Britannica Syndication API Documentation

Table of Contents

1. Overview
   1. Introduction
   2. Terminology
2. Britannica Syndication API
   1. Introduction to the API
      1. Overview of Available Products
         1. Encyclopaæia Britannica
         2. Britannica Concise Encyclopedia
         3. Britannica Intermediate Encyclopedia
         4. Britannica Elementary Encyclopedia (US English)
         5. Britannica Primary Encyclopedia (UK English)
         6. Portuguese Primary Encyclopedia
         7. Portuguese custom articles
         8. Thematic Products
      2. Overview of API Site
      3. Viewing an Article List
      4. Viewing an Article in XML
      5. Common Errors
   2. Using the API in Development
      1. Extracting an Article List in JSON: /articles
      2. Extracting Article Content in XML: /article/{articleId}/xml
      3. Accessing API Metadata in JSON: /application/metadata
3. Britannica Syndication XML Data Format
   1. Syndication XML DTD
   2. Article Structure and Organization
      1. Article: article
      2. Title of Article, Section, or Media Assembly: title
      3. Section Levels: h1, h2, h3, h4, h5, h6
      4. Paragraph: p
      5. List and List Item: list, item
      6. List Heading: listhead
      7. Media Assembly: assembly
      8. Media Item: media
      9. Media Caption and Credit: caption, credit
      10. Inline Media Item: inlinemedia
      11. Quote: quote
      12. Poetry: poetry, line
      13. Stanza of Poetry: linegroup
      14. Author of Poem: attribution
      15. Table and Table Body: table, tbody
      16. Table Header and Footer: thead, tfoot
      17. Table Row and Table Cell: tr, td
      18. Biological Classification: classification, taxon1, taxon2, taxon3, taxon4, taxon5, taxon6
      19. Classification Label and Description: label, taxonname, commonname, desc
      20. Author of Article or Section: signature
      21. Copyright Notice: copyright
   3. Cross-References
      1. Internal Cross-Reference: xref
      2. External URL: link
      3. Reference to Assembly: assemblyref
      4. Inline Media Reference: inlinemediaref
      5. Table Reference: tableref
   4. Text Formatting
      1. Emphasis (Bold, Italic, Bold-Italic, Smallcaps, Underline): e
      2. Superscript: sup
      3. Subscript: sub
      4. Overbar: overbar
      5. Formula: formula
      6. Line Break: linebreak
      7. Fractions: fraction, num, den
      8. Integrals: integral, lower, upper
      9. Square Root: sqroot

1. Overview

Introduction

This reference guide to the Britannica Syndication API explains how to use the API website to explore and extract data for Britannica syndication products. It also provides a detailed definition of the XML markup of encyclopedia articles and the JSON output of API requests.

The Terminology section contains a helpful glossary of terms used in discussing Britannica article content, structure, and extraction.

Section 2, Britannica Syndication API, explains how to use the API website to become familiar with Britannica content and evaluate the available encyclopedia products. For developers, it also provides a detailed description of JSON and XML extraction methods, and addresses how the two data formats interact for production purposes.

Section 3, Britannica Syndication XML Data Format, explains the rich semantic, organizational, and formatting markup of the article content. The description of each XML element is broken into four sections:

• The Usage Context section describes where and how an XML element is used, while the Data Model section details its contents and attributes. These two sections summarize the Britannica DTD's validity rules in prose, but also expand on them by addressing processing issues and clarifying the semantics and interrelationships of data elements.
• The Display Guidance and Discussion section provides guidance on how the element is typically rendered, and addresses important points of note or caution on content handling.
• The XML Example section provides one or more examples of the element in actual use, along with an explanatory preface.

Mentions of JSON code and XML elements and attributes are in a small reddish font throughout the document. When a JSON type is discussed (eg, object, string, Boolean), a small blue font is used.

Terminology

2. Britannica Syndication API

Introduction to the API

This section is intended as a guide to evaluating the products available in the Syndication API, and does not assume prior knowledge of the API's underlying technologies. If you are already familiar with JSON, XML, and the Swagger documentation tool underlying the API site, you may want to read the Overview of Available Products, then jump ahead to Using the API in Development.

Developers can interact with the Britannica Syndication API with code, but anyone can test it out without the need to write software using the API website. This site lets you interact with the API and sample our available products via a visual interface.

Any form of API access requires an authorization key. If you are interested in evaluating the API, please register for an account to obtain a key. When registering, select the product for which you would like to receive a key from the “Article Type” drop-down menu. If you are only interested in articles in a specific subject area, select one from the “Category” drop-down menu. If you need to evaluate more than one product or thematic product, please contact us to obtain additional authorization keys.

Overview of Available Products

Our API features products in multiple languages serving a wide range of audiences. While the products vary in scope, depth, and reading level, all of them cover a broad, general range of topics appropriate for the target audience. By contrast, our thematic products are subsets of the main products, each focusing on a specific subject area.

The following products and thematic products are available via the Britannica Syndication API. As you read the descriptions below, please make note of the articleTypeId number given for your product, and, if you are evaluating a thematic product, the categoryId number as well. You will use this information later to view the article list.

Products

Encyclopædia Britannica

• Our premier encyclopedia, covering a world of topics with extensive articles by Britannica's expert team of professional editors. Contains the same core content featured on Britannica.com.
• Over 87,000 articles | 405 MB XML | over 47,000 images | 2,100 video and audio clips
• articleTypeId: 1

Britannica Concise Encyclopedia

• Covers a broad range of subjects in concise articles that quickly summarize the key information you need to grasp a topic. Ideal for use cases where you need to convey the essentials in limited space or supplement other information.
• Over 25,000 articles | 29 MB XML data
• articleTypeId: 45

Britannica Intermediate Encyclopedia

• Designed for middle grade and high school students, this encyclopedia combines broad coverage of topics with in-depth information suitable for classroom, homework, and research.
• Over 22,000 articles | 98 MB XML | over 23,000 images | 1,000 video and audio clips
• articleTypeId: 31

Britannica Elementary Encyclopedia

• Designed for elementary school students grades 3-5, this encyclopedia covers all the essential topics in an accessible style. Written in US English, this content is featured on the Britannica Kids website.
• Over 4,800 articles | 16 MB XML | 6,900 images | 550 video and audio clips
• articleTypeId: 38

Britannica Primary Encyclopedia

• Designed to provide primary school students with detailed information on all the essential topics. Written in British English, this encyclopedia covers the same subjects as the Elementary Encyclopedia for the US market but features more extensive articles.
• Over 4,800 articles | 19 MB XML | 6,100 images | 400 video and audio clips
• articleTypeId: 94

Portuguese Primary Encyclopedia

• Portuguese-language encyclopedia for primary school students.
• Over 2,900 articles | 13 MB XML | 7,100 images | 400 video and audio clips
• articleTypeId: 108

Portuguese custom articles

• Customized Portuguese-language encyclopedia articles
• articleTypeId: 153

Thematic Products

A thematic product covers a particular subject area using a subset of articles from one of the products above. You customize this content intersection for your desired audience; for instance, you could have biographical articles targeted at middle-grade students (articleTypeId 31, categoryId 3) or science articles with in-depth, high-level content (articleTypeId 1, categoryId 2). Article count and data size will vary based on your particular combination of source product and subject area.

• Britannica Encyclopedia of Animals
  • categoryId: 9
• Britannica Encyclopedia of Art
  • categoryId: 1
• Britannica Encyclopedia of Events
  • categoryId: 5
• Britannica Encyclopedia of People
  • categoryId: 3
• Britannica Encyclopedia of Places
  • categoryId: 7
• Britannica Encyclopedia of Plants
  • categoryId: 6
• Britannica Encyclopedia of Science
  • categoryId: 2
• Britannica Encyclopedia of Sports
  • categoryId: 4
• Britannica Encyclopedia of Technology
  • categoryId: 8

Overview of API Site

1. The first step when visiting the API site is to enter your authorization key. If you do not yet have a key, please register for an account to obtain one.

On the right near the top of the page, you will see a button containing the word "Authorize" and an image of an open padlock:

2. Click on the "Authorize" button. A pop-up will appear. Type your authorization key in the box, then click the green "Authorize" button. Click "Close" to dismiss the pop-up.

When you return to the main page, the "Authorize" button will now show the image of a closed padlock:

3. Scroll down until you see the Content heading with blue "GET" buttons underneath it.

Each blue section with a GET button is referred to as an endpoint.

4. The first endpoint, /articles, retrieves a list of articles matching parameters you define. Click on the blue bar for the endpoint, which then expands to this view:

Click the "Try it out" button at right, then go to Viewing an Article List for more details on how to refine and navigate the article list.

5. The second endpoint, /article/{articleId}/xml, retrieves the content of a particular article in XML. Click on the blue bar for the endpoint, which then expands to this view:

Click the "Try it out" button at right, then go to Viewing an Article in XML for more details on how to retrieve a particular article from the API.

Viewing an Article List

This API function lets you retrieve an encyclopedia's article list, which gives you a sense of the topics covered and provides key information needed to look up individual articles. There are four parameters you can set: articleTypeId, categoryId, lastUpdated, and page. In the steps below, we will try out the first and third parameters with the /articles endpoint, then learn how to use the fourth to page through the results. Finally, we will see how to retrieve a thematic subset of articles using categoryId.

NOTE: your access to particular products and thematic products is limited by the authorization key(s) you requested when registering for an account. As a result, you may not be able to replicate all of the steps described below. If you need to evaluate additional products, please contact us to obtain additional authorization keys.

1. Under the "Content" heading, look for the blue "GET" button followed by the text /articles. Click on that "GET" button.

2. In the first box, enter the articleTypeId number (found here) of the product you want to examine.

3. In the third box, enter a lastUpdated date about 3 months before today's date, using the format YYYY-MM-DD. The lastUpdated parameter lets you see which articles have been modified since the date you provide. Here we will use it to find articles that have been updated to reflect recent events.

In the example below, the first and third boxes narrow the search to articles in our largest encyclopedia that have been modified since Nov 1, 2021 (recent at the time of writing):

4. Now click on the blue "Execute" button and scroll down to "Server response", where you'll find the article list.

The "totalCount" item at the top tells us the number of articles that have been updated since the date provided. You can scroll through the list in the results pane, or click on the "Download" button at bottom right to save a copy. When you find an article that interests you, make a note of the number after "articleId", as you will use it to view the article in the next section.

5. In the example above, totalCount tells us that 1815 articles have been recently updated. The page parameter lets us specify which "page" of results we want to view, where each page is a list of 1000 articles. By default, we just see the first page.

To see the remaining 815 articles, we scroll back up, type "2" in the box next to "page", then rerun the search by clicking "Execute". We then see a different set of articles:

6. The new articles listed indicate you are viewing a new page of up to 1000 articles.

Note the totalCount value has not changed, as we did not change articleTypeId and lastUpdated when rerunning the search, and these parameters alone determine the overall results. Entering a value for page specifies which page we're viewing within a defined set of results, but doesn't change the search results themselves.

7. Finally, we will modify our search to view the article list for a thematic product. Enter "31" for articleTypeId in the first box and "9" for categoryId in the second box. This narrows the results to articles on animals within the Britannica Intermediate Encyclopedia product.

8. Now click on the blue "Execute" button and scroll down to "Server response" to view the article list. You will see there are 730 articles on animals that comprise this particular thematic product.

After you browse or download this article list, return to the previous step and enter a new categoryId number (found here) to view articles on a different subject.

Viewing an Article in XML

This API function lets you view the content of an individual article in XML format. This search only requires you to input the article's ID number, or articleId. Please refer to the article list you generated in the previous section, and make note of one or more articleId numbers that you're interested in looking up.

1. Under the "Content" heading, look for the blue "GET" button followed by the text /article/{articleId}/xml. Click on that "GET" button.

2. In the box, enter the articleId number for the article you wish to view. You can get this number from the article list you viewed in the previous section.

3. Click on the blue "Execute" button and scroll down to "Server response". You can scroll through the article in the results pane, or click on the "Download" button at bottom right to save a copy.

Common Errors

When an error occurs during a search, you will see a message like this under the "Server response" heading:

Here are the error codes that can occur and their typical causes:

400: This code means you entered an articleId that does not actually exist during an XML article search. Check your article list, then reenter a valid articleId.

401: This code appears when you do not have permission to view the content you've requested. There are several possible causes of this:

1. You have not entered an authorization key. If you do not have one yet, you need to register for an account first. Once you have your key(s), click on the padlock icon to enter the key for the product you wish to view.
2. You mistyped your authorization key. Click on the padlock icon, click "Logout", then reenter your authorization key.
3. You selected a product under articleTypeId that does not match the active authorization key. Check to make sure your key and the product match, then adjust as needed.

Using the API in Development

Extracting an Article List in JSON: articles

The article list is the key to developing a Britannica syndication product. In this section we describe how to extract and interpret a product or thematic product article list, whether full or partial (for instance, a subset of articles that have been updated since a particular date). The article list is then used as input in extracting the relevant XML articles.

NOTE: For thematic products, the article list may also be needed in processing the XML content -- please see discussion under Internal Cross-References.

JSON Example

Output of /articles GET operation. The parameter articleTypeId is set to "38" (elementary level). The parameters categoryId and lastUpdated are not set, while page uses the default value.

{
  "totalCount": 4888,
  "articles": [
    {
      "articleId": 345449,
      "articleTypeId": 38,
      "title": "John Adams",
      "lastUpdated": "2021-08-04"
    },
    {
      "articleId": 345450,
      "articleTypeId": 38,
      "title": "John Quincy Adams",
      "lastUpdated": "2021-08-04"
    },
    {
      "articleId": 345451,
      "articleTypeId": 38,
      "title": "Alabama",
      "lastUpdated": "2021-08-05"
    },
    {
      "articleId": 345452,
      "articleTypeId": 38,
      "title": "Alaska",
      "lastUpdated": "2021-08-05"
    },
    [...]
}

Extracting Article Content in XML: /article/{articleId}/xml

Britannica encyclopedia content is provided in the Britannica Syndication XML format. Once you have extracted your article list in JSON as described in the previous section, you can use code to extract an entire set of XML articles, or simply retrieve an individual article from the API site.

XML Example

On the API site, an example of the XML output for articleId "2350". Note the XML attribute articletypeid, which indicates the product to which the article belongs.

Accessing API Metadata in JSON: /application/metadata

This endpoint provides the metadata for the API.

3. Britannica Syndication XML Data Format

Syndication XML DTD

All of the XML articles in Syndication API products and thematic products are validated against the DTD syndication_xml.dtd, which can be accessed here.

Article Structure and Organization

Article: article

The article is the organizational unit of an encyclopedia product or thematic product. Each article is contained in a single XML file.

XML Example

The article element encompassing all article elements and text, from "threonine", ArticleId 100004.

<article articleid="100004" articletypeid="1" version="4" language="en_us"
 lastupdate="2021-Aug-06 10:12:57" url="/article/threonine/100004">
 <title>threonine</title>
 <p><e type="bold">threonine</e>, an <xref articleid="7182">amino acid</xref>
 obtainable from many <xref articleid="110575">proteins</xref>. One of the
 last amino acids to be isolated (1935), threonine is one of several so-called 
 essential amino acids; i.e., animals cannot synthesize it and require dietary
 sources. It is synthesized in microorganisms from the amino acid 
 <xref articleid="9891">aspartic acid</xref>. The chemical
 structure of threonine is
  <formula>
   <inlinemedia id="i124007" mediaid="124007" url="/07/124007-004-58B81A75.jpg"/>
  </formula>
 </p>
 <copyright>Encyclopædia Britannica</copyright>
</article>

Title of Article, Section, or Media Assembly: title

The title element's presentation and content are key to a user's understanding of an article. The title defines the main topic of the article as well as the subject headings for the article's major organizational sections.

XML Example

The article "Italy" makes use of title in all possible contexts. In the first instance below, title defines the topic of the entire article. The article's deep hierarchical structure relies on title to provide each section's heading, as seen with h2 and h3. Finally, in the media assembly the title provides context for the video clip presented to the user.

<article articleid="106448" articletypeid="1" version="93"
 language="en_us" lastupdate="2021-Sep-17 08:46:43"
 url="/article/Italy/106448">
  <title>Italy</title>
  [...]
  <h2 id="h258785">
    <title>Drainage</title>
    <h3 id="h26964">
      <title>Rivers</title>
      <p>
        [...]
        <assembly id="a16674" url="/assembly/view/16674">
          <title>Watch Italy’s Po River flow through the Piedmont
           region and drain into the fertile Po River valley</title>
          <media mediaid="21929" type="video" url=""
           thumburl="/29/21929-049-CF36BCB7.jpg"/>
          <caption>Views of the Po River, Italy, and its fertile
           valley.
          </caption>
          <credit>Encyclopædia Britannica, Inc.</credit>
        </assembly>
        [...]
      </p>
    [...]
    </h3>
  [...]
  </h2>
  [...]
</article>

Section Levels: h1, h2, h3, h4, h5, h6

A section level encloses a major section of an article. It is part of a hierarchy with h1 at the top level, h2 a subsection of h1, and so on down to the lowest level of h6.

Unlike the HTML elements with the same names, here h1-h6 encompass an entire section's content, not just the section headings. In Britannica Syndication XML, the heading for a particular section level is marked up by title.

XML Example

A sequence of nested h1, h2, h3 elements in the article "Korean literature", ArticleId 106467.

<h1 id="h284734">
 <title>History</title>
 <h2 id="h284735">
  <title>Classical literature</title>
  <h3 id="h284736">
   <title>Ancient times</title>
   <p>The origins of Korean literature can be traced back to an Old Stone
   Age art form that combined dance, music, and literature. Originating
   in festival activities, this art form served the political function of
   unifying society, the religious function of identifying and describing
   a supernatural power capable of averting calamity on earth, and the
   economic function of inspiring productive activity, especially that
   related to agriculture. The farming and work songs and the early forms
   of myth and narrative poetry that had their basis in the abundant harvests
   of the New Stone Age were probably transmitted orally; it is difficult
   to find examples that have survived intact.</p>
   [...]
  </h3>
  [...]
 </h2>
 [...]
</h1>

Paragraph: p

This element always contains what its name implies: a paragraph of text. However, it also serves to organize important structural elements of an entry—notably assembly—which may or may not be desirable to render in place, depending on the implementation.

XML Example

A p element containing text, but also initial assembly and assemblyref elements, from the article "copper", articleId 26194.

<p>
 <assembly id="a222338" url="/assembly/view/222338">
  <title>copper cables</title>
  <media mediaid="157765" type="image" url="/65/157765-004-2C78D66A.jpg"/>
  <caption>Copper electrical cables. Because of copper’s high electrical
   conductivity, it is heavily used in the electrical industry.</caption>
  <credit>© Pegasus/Fotolia</credit>
 </assembly>
 <assemblyref assemblyid="a222338"/>
 Copper is one of the most ductile metals, not especially strong or hard. 
 Strength and <xref articleid="472487">hardness</xref> are appreciably 
 increased by cold-working because of the formation of elongated 
 <xref articleid="110297">crystals</xref> of the same face-centred cubic
 structure that is present in the softer annealed copper. Common 
 <xref articleid="110302">gases</xref>, such as 
 <xref articleid="57841">oxygen</xref>, <xref articleid="55947">nitrogen</xref>, 
 <xref articleid="20249">carbon dioxide</xref>, and <xref articleid="473414">
 sulfur dioxide</xref> are soluble in molten copper and greatly affect 
 the mechanical and electrical properties of the solidified metal. The
 pure metal is second only to <xref articleid="67807">silver</xref>
 in thermal and <xref articleid="628185">electrical conductivity</xref>.
 Natural copper is a mixture of two stable 
 <xref articleid="110412">isotopes</xref>: copper-63 (69.15 percent)
 and copper-65 (30.85 percent).
</p>

List and List Item: list, item

A list consists of a series of list items, each preceded by the appropriate lead-in character.

XML Example

An example of an unordered list and list item elements in the article "The Clash", articleId 117781.

<h1 id="h93037">
 <title>Major Works</title>
 <list type="ul">
  <item><e type="italic">The Clash</e> (U.K. 1977, U.S. 1979)</item>
  <item><e type="italic">Give ’Em Enough Rope</e> (U.K. 1978,
   U.S. 1979)</item>
  <item><e type="italic">London Calling</e> (U.K. 1979,
   U.S. 1980)</item>
  <item><e type="italic">Combat Rock</e> (1982)</item>
  <item><e type="italic">Clash on Broadway</e> (1991)</item>
 </list>
</h1>

List Heading: listhead

A heading at the beginning of a list that identifies its subject or provides context.

XML Example

Use of listhead to specify subjects of lists, from the article "Greek language", articleId 109772.

<list type="ul">
 <listhead><e type="italic">West Group (Doric in the widest
  sense)</e></listhead>
 <item>(1) North-West Greek: Aetolia, Locris (colony—Locri
  Epizephyrii), Phocis, </item>
 <item>(2) Saronic Doric: the territory of Corinth (colonies—
  Corcyra, Syracuse), the Megarid (colonies—Megara Hyblaea, Selinus,
  Byzantium), Eastern Argolid, </item>
 <item>(3) Western Argolid, </item> 
 <item>(4) South-East Aegean Doric: Melos and Thera (colony—
  Cyrene), Cos, Rhodes (colonies—Gela, Acragas), </item>
 <item>(5) Crete, </item>
 <item>(6) Laconia (colonies—Tarentum, Heraclea), Messenia, </item>
 <item>(7) Achaea (colonies—Ithaca, Sybaris), </item>
 <item>(8) Elis.</item>
</list>
<list type="ul">
 <listhead><e type="italic">Aeolic Group</e></listhead>
 <item>(1) Boeotia, </item>
 <item>(2) Thessaly, </item>
 <item>(3) Lesbos and Asiatic Aeolis.</item>
</list>

Media Assembly: assembly

A media assembly brings together a media item (an image, audio clip, or video) and text that provides context or a creative credit for the media.

XML Example

An assembly at the beginning of the article "marsupial", articleId 105974.

<title>marsupial</title>
<p>
 <assembly id="a145430" url="/assembly/view/145430">
  <title>red kangaroo (<e type="italic">Macropus rufus</e>)</title>
  <media mediaid="140490" type="image" url="/90/140490-004-23FE0B09.jpg"/>
  <caption>Red kangaroo (<e type="italic">Macropus rufus</e>).
   The home range of this species spans much of Australia’s interior, and it is
   the largest member of the family Macropodidae.</caption>
  <credit>© Anup Shah—Photodisc/Thinkstock</credit>
 </assembly>
[...]
</p>

Media Item: media

An assembly's media element contains the specific information required to render a media item to users, such as its URL and media type (ie, image, audio clip, or video).

XML Example

Two media assemblies in the article for "Wayne Gretzky" (articleId 1584), one with a media element referencing an image and another pointing to a video. Note that the thumburl attribute is only present in the media element referencing a video.

<assembly id="a227361" url="/assembly/view/227361">
  <title>Wayne Gretzky</title>
  <media mediaid="193197" type="image" url="/97/193197-004-17F30664.jpg"/>
  <caption>Wayne Gretzky.</caption>
  <credit>© Jerry Coli/Dreamstime.com</credit>
</assembly>
[...]
<assembly id="a226133" url="/assembly/view/226133">
  <title>Discover the sizable gap between the Great One’s career-point
   record and Jaromir Jagr’s second-place totals</title>
  <media mediaid="194293" type="video" url="Oz0lTSrX"
   thumburl="/93/194293-049-C628D4A6.jpg"/>
  <caption>Learn about the career of hockey player
   Wayne Gretzky.</caption>
  <credit>Encyclopædia Britannica, Inc.</credit>
</assembly>

Media Caption and Credit: caption, credit

The caption identifies and provides context for the assembly's media item, while the credit gives information on the media's creator and/or copyright holder.

XML Example

A caption providing explanatory information for an image along with a credit in the article "marsupial", articleId 105974.

<assembly id="a4422" url="/assembly/view/4422">
 <title>long-nosed bandicoot (<e type="italic">Perameles nasuta</e>)</title>
 <media mediaid="6262" type="image" url="/62/6262-004-C4E9A25F.jpg"/>
 <caption>Long-nosed bandicoot (<e type="italic">Perameles nasuta</e>).
  The species can be found in the coastlands of eastern Australia, from the
  Cape York Peninsula south to Victoria.</caption>
 <credit>Warren Garst—Tom Stack and Associates</credit>
</assembly>

Inline Media Item: inlinemedia

An inlinemedia element represents an image within the flow of article content, outside of an assembly structure. It contains specific information required to render the image to users and refer to it from other locations within the article.

XML Example

Use of inlinemedia within a formula in the article "zinc processing", articleId 110671.

<h2 id="h82101">
 <title>Roasting and sintering</title>
 <p>Both of the main extraction methods for the production 
  of zinc, electrolysis and smelting, require the prior removal
  of sulfur in a highly exothermic oxidation reaction:
  <formula>
   <inlinemedia id="i15496" mediaid="15496" url="/96/15496-004-D103AECD.jpg"/>
  </formula>
 </p>
 [...]
</h2>

Quote: quote

The quote element marks a quotation from a cited source.

XML Example

Use of quote in the article "historiography", articleId 108622.

<p>By about 710 <e type="smallcap">ce</e>, however,
 Liu Zhiji (661–721) had produced the <e type="italic">Shitong</e>
 (“Historical Perspectives”), the first comprehensive work on historical
 criticism in any language. For him, the writing of history had an
 exalted—and very Confucian—mission:
</p>
<quote>
 <p>Man lives in his bodily shape between heaven and earth and
  his life is like the span of the summer fly, like the passing of a
  white colt glimpsed through a crack in the wall. Yet he is shamed
  to think that within those years his merit will not be known…there
  is truly none who is not tireless in pursuing merit and fame.…Why
  is this? Because all have their heart set on immortality. And what,
  then, is immortality? No more than to have one’s name written
  in a book.
 </p>
</quote>

Poetry: poetry, line

These two elements supply the most basic formatting for poems within encyclopedia articles: poetry encloses an entire poem, while line surrounds each line of the poem.

XML Example

Use of poetry and line to format the poem "Directive" by Robert Frost in the article "feminine ending", articleId 125581.

<poetry>
 <line>Back out of all this now too much for us,</line>
 <line>Back in a time made simple by the loss</line>
 <line>Of detail, burned, dissolved, and broken off </line>
 <line>Like graveyard marble sculpture in the weather, </line>
 <line>There is a house that is no more a house</line>
 <line>Upon a farm that is no more a farm </line>
 <line>And in a town that is no more a town.</line>
</poetry>

Stanza of Poetry: linegroup

The element linegroup is used to identify a stanza, or group of lines set apart in a poem.

XML Example

Stanzas marked with linegroup in the article "Auld Lang Syne", articleId 628262.

<title>Scots version</title>
<poetry>
 <linegroup>
  <line>Should auld acquaintance be forgot,</line>
  <line>And never brought to mind?</line>
  <line>Should auld acquaintance be forgot,</line>
  <line>And auld lang syne?</line>
 </linegroup>
 <linegroup>
  <line>(Chorus)</line>
  <line>For auld lang syne, my jo,</line>
  <line>For auld lang syne,</line>
  <line>We’ll tak a cup o’ kindness yet,</line>
  <line>For auld lang syne.</line>
 </linegroup>
[...]
</poetry>

Author of Poem: attribution

An attribution identifies the author of a poem contained in a poetry element. Other source information may be included in the attribution as well.

This element should not be confused with signature, which is the author attribution for article content.

XML Example

Use of attribution to identify a poem's author in the article "English literature", articleId 106051.

<p>[...] While modern mechanical technologies were pressed into
service by the Stuarts to create the scenic wonders of the court
<xref articleid="51282">masque</xref>, the discoveries of astronomers and
explorers were redrawing the cosmos in a way that was
profoundly disturbing:</p>
<poetry>
 <line>And freely men confess that this world’s spent,</line>
 <line>When in the planets, and the firmament</line>
 <line>They seek so many new….</line>
 <attribution>(John Donne, <e type="italic">The First Anniversary</e>,
  1611)</attribution>
</poetry>

Table and Table Body: table, tbody

A table contains a set of information meant to be presented in rows and columns. The tbody element surrounds the body of the table, in contrast to the header and footer. The body consists of a series of table rows, each of which contains table cells.

XML Example

A simple use of table and tbody in the article "English language", articleId 109779.

<p>In standard Old English, adjectives, nouns,
 pronouns, and verbs were fully inflected. [...] Personal, possessive,
 demonstrative, interrogative, indefinite, and relative pronouns had
 full inflections. The pronouns of the 1st and 2nd persons still had
 distinctive dual forms:</p>
<table>
 <tbody>
  <tr>
   <td><e type="italic">iċ</e></td>
   <td>“I”</td>
   <td><e type="italic">wit</e></td>
   <td>“we two”</td>
   <td><e type="italic">wē</e></td>
   <td>“we”</td>
  </tr>
  <tr>
   <td><e type="italic">thū (þū)</e></td>
   <td>“thou”</td>
   <td><e type="italic">ġit</e></td>
   <td>“you two”</td>
   <td><e type="italic">ġē</e></td>
   <td>“you”</td>
  </tr>
 </tbody>
</table>

Table Header and Footer: thead, tfoot

The table header thead contains one or more header rows for a table, displayed before the main table body to identify the type of information contained in each table cell. Similarly, the table footer tfoot occurs after the table body and contains the table's footnotes.

XML Example

A table with both thead and tfoot sections in the article "vaccine", articleId 74606.

<table>
 <thead>
  <tr>
   <td>disease</td>
   <td>year</td>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>smallpox<sup>*</sup></td>
   <td>1798<sup>**</sup></td>
  </tr>
  <tr>
   <td>rabies</td>
   <td>1885<sup>**</sup></td>
  </tr>
  <tr>
   <td>typhoid</td>
   <td>1896<sup>**</sup></td>
  </tr>
  <tr>
   <td>cholera</td>
   <td>1896<sup>**</sup></td>
  </tr>
  [...]
  <tr>
   <td>dengue fever</td>
   <td>2019</td>
  </tr>
 </tbody>
 <tfoot>
  <tr>
   <td><sup>*</sup>Vaccine recommended for universal use in 
    U.S. children. For smallpox, routine vaccination was ended in 1971.</td>
  </tr>
  <tr>
   <td><sup>**</sup>Vaccine developed (i.e., first published
    results of vaccine usage).</td>
  </tr>
  <tr>
   <td><sup>***</sup>Vaccine licensed for use in United
    States.</td>
  </tr>
 </tfoot>
</table>

Table Row and Table Cell: tr, td

Tables present information in rows and columns. An individual table row is contained in a tr element, and consists of a series of table cell td elements. The count of cells in each row of the table header and table body should match, and determines the number of columns in the table.

XML Example

Table rows tr and cells td in a simple two-column table in the article "helium", articleId 1713.

<table class="element">
 <tbody>
  <tr>
   <td>atomic number</td>
   <td>2</td>
  </tr>
  <tr>
   <td>atomic weight</td>
   <td>4.002602</td>
  </tr>
  <tr>
   <td>melting point</td>
   <td>none</td>
  </tr>
  <tr>
   <td>boiling point</td>
   <td>−268.9 °C (−452 °F)</td>
  </tr>
  <tr>
   <td>density (1 atm, 0 °C)</td>
   <td>0.1785 gram/litre</td>
  </tr>
  <tr>
   <td>oxidation state</td>
   <td>0</td>
  </tr>
  <tr>
   <td>electron configuration</td>
   <td>1<e type="italic">s</e><sup>2</sup></td>
  </tr>
 </tbody>
</table>

Biological Classification/Taxonomy: classification, taxon1, taxon2, taxon3, taxon4, taxon5, taxon6

The biological classification relates the common name subject of an article (eg, "owl") to its scientific taxonomy (eg, "Order Strigiformes, Family Tytonidae, Family Strigidae"), providing detailed nomenclature and descriptive information for each level of the hierarchy. The classification element surrounds the entire taxonomy. The highest level discussed is captured by the taxon1 element, with subsequent levels represented by taxon2, taxon3, taxon4, taxon5, and taxon6. However, only a small number of articles use the full set of elements.

Note that the top level of the taxonomy found in taxon1 is not fixed, but varies by article. For instance, the article "owl" has the Order at the top level, while the taxonomy in "vertebrate" begins with the Subphylum.

XML Example

A classification and taxon1-taxon6 hierarchy in the article "branchiopod", articleId 110243.

<h2 id="h33747">
 <title>Annotated classification</title>
 [...]
 <classification>
  <taxon1>
   <label>
    <taxonname>Phylum Arthropoda</taxonname>
   </label>
   <taxon2>
    <label>
     <taxonname>Subphylum Crustacea</taxonname>
    </label>
    <taxon3>
     <label>
      <taxonname>Class Branchiopoda</taxonname>
     </label>
     [...]
     <taxon4>
      <label>
       <taxonname>Order Diplostraca</taxonname>
      </label>
      [...]
      <taxon5>
       <label>
        <taxonname>Suborder Cladocera</taxonname>
       </label>
       <taxon6>
        <label>
         <taxonname>Infraorder Ctenopoda</taxonname>
        </label>
        <desc>Short-bodied forms with 6 pairs of trunk limbs, of
         which 5 bear filters; bivalved carapace encloses trunk but not
         head; antennae large, used in swimming, and bearing long swimming
         setae; all filter feeders; no larval stages, young hatch as
         miniatures of adult; worldwide in fresh water, except Antarctica;
         one genus, <e type="italic">Penilia</e>, is marine.
        </desc>
       </taxon6>
       [...]
      </taxon5>
      [...]
     </taxon4>
     [...]
    </taxon3>
   </taxon2>
  </taxon1>
 </classification>
</h2>

Classification Label and Description: label, taxonname, commonname, desc

The label contains names for a particular taxonomic level in a biological classification. The taxonomic name is always given in a taxonname element; a common name commonname may be provided as well.

The desc element provides a detailed description that accompanies the label.

XML Example

In the article "owl", articleId 105937, several sets of descriptions desc and labels label, some of which have both taxonname and commonname children.

<classification>
 <taxon1>
  <label>
   <taxonname>Order Strigiformes</taxonname>
  </label>
  <desc>180 species in 2 families of nocturnal predatory birds;
   hooked beak, broad skull, facial disk with forward-directed eyes;
   reversible outer toe; talons for securing prey.
  </desc>
  <taxon2>
   <label>
    <taxonname>Family Tytonidae </taxonname>
    <commonname>(<xref articleid="13399">barn owl</xref>s,
     grass owls, and <xref articleid="13851">bay owl</xref>s)
    </commonname>
   </label>
   <desc>21 species in 2 genera found from tropical to temperate
    regions; body length 30–54 cm (12–21 inches); heart-shaped facial
    disk completely encircling face, bill comparatively long and slender;
    legs rather long; middle claw with comb.
   </desc>
  </taxon2>
  <taxon2>
   <label>
    <taxonname>Family Strigidae </taxonname>
    <commonname>(<xref articleid="18228">burrowing owl</xref>,
     <xref articleid="31701">eagle owl</xref>, <xref articleid="32375">
     elf owl</xref>, <xref articleid="34386">fish owl</xref>,
     <xref articleid="39606">hawk owl</xref>,
     <xref articleid="41082">horned owl</xref>s,
     <xref articleid="48545">little owl</xref>,
     <xref articleid="48854">long-eared owl</xref>,
     <xref articleid="62019">pygmy owl</xref>,
     <xref articleid="66382">screech owl</xref>,
     <xref articleid="67502">short-eared owl</xref>,
     <xref articleid="68408">snowy owl</xref>,
     <xref articleid="69033">spectacled owl</xref>,
     <xref articleid="77397">wood owl</xref>, and others)
    </commonname>
   </label>
   <desc>159 species in 20 genera found nearly worldwide;
    length 13–70 cm (5–28 inches).
   </desc>
  </taxon2>
 </taxon1>
</classification>

Author of Article or Section: signature

The signature element identifies the author of part or all of an article's content. It is very flexible in its placement within the article structure, as there may be one or more authors for the article as a whole, as well as authors credited for individual sections (h1, h2, etc) or a series of paragraphs.

XML Example

Uses of signature at the end of an h2 and an h1, from the article "nitrogen group element", articleId 110610.

<article articleid="110610" articletypeid="1" version="14" language="en_us"
 lastupdate="2021-Jul-01 04:50:54" url="/article/nitrogen-group-element/110610">
 <h1 id="h80985">
  <title>Comparative chemistry</title>
  [...]
  <h2 id="h80991">
   <title>Comparison of nitrogen group elements</title>
   [...]
   <signature>R. Thomas Sanderson</signature>
  </h2>
 </h1>
 <h1 id="h81027">
  <title>Bibliography</title>
  [...]
  <signature>R. Thomas Sanderson</signature>
  <signature>EB Editors</signature>
 </h1>
 <copyright>Encyclopædia Britannica</copyright>
</article>

Copyright Notice: copyright

Provides the copyright information for the article.

XML Example

The copyright notice for the article "curium", articleId 28258. Note the placement of copyright as the last element in the article.

<article articleid="28258" articletypeid="1" version="9" language="en_us"
 lastupdate="2021-Jun-30 21:27:01" url="/article/curium/28258">
 <title>curium (Cm)</title>
 <p>[...] <e type="bold">curium (Cm)</e>, <assemblyref
  assemblyid="a64371"/>synthetic <xref articleid="110602">chemical
  element</xref> of the <xref articleid="110616">actinoid series
  </xref> of the periodic table, [...]</p>
 [...]
 <signature>Lester Morss</signature>
 <table class="element"> [...] </table>
 <copyright>Encyclopædia Britannica</copyright>
</article>

Cross-References

Internal Cross-Reference: xref

A cross-reference to an article, a specific section within the current article, or a specific section in a different article.

XML Example

A two-part example, showing:

(1) An xref in the article "gravity", articleId 106265, that targets a specific section via refid within an article specified by articleid.

<p>The prime example of a field theory is Einstein’s 
 <xref articleid="109465" refid="h252889">general relativity</xref>,
 according to which the acceleration due to gravity is a purely geometric
 consequence of the properties of space-time in the neighbourhood of
 attracting masses. [...]
</p>

(2) The target article "relativity", articleId 109465, where an h1 section level has an id corresponding to the refid in (1).

<h1 id="h252889">
 <title>General relativity</title>
 <h2 id="h252890">
  <title>Roots of general relativity</title>
  <p>Because <xref articleid="108764">Isaac Newton</xref>’s
   law of <xref articleid="106265">gravity</xref> served so well in
   explaining the behaviour of the solar system, the question arises why it was
   necessary to develop a new theory of gravity. The answer is that Newton’s
   theory violates special relativity, for it requires an unspecified “action
   at a distance” through which any two objects—such as the Sun and Earth—
   instantaneously pull each other, no matter how far apart. [...]
  </p>
  [...]
 </h2>
 [...]
</h1>

External URL: link

A reference to a URL outside of Britannica API content.

Reference to Assembly: assemblyref

A cross-reference to, or marker of appropriate insertion point for, an assembly element.

See also Media Assembly for more information.

XML Example

Example (1): An empty assemblyref pointing to the immediately preceding assembly in the article "marsupial", articleId 105974.

<p>
 <assembly id="a147045" url="/assembly/view/147045">
  <title>forester, or eastern gray kangaroo</title>
  [...]
 </assembly>
 <assemblyref assemblyid="a147045"/>
 There seems to be little permanent social organization among most marsupials
 beyond short-lived pair bonds during mating. Many of the grazing marsupials,
 such as kangaroos and wallabies, move in feeding groups called mobs, but those
 associations are not true social groups, as there is no attention paid to any
 leaders or elders.
 [...]
</p>

Example (2): An assemblyref containing reference text pointing to the assembly at the start of the paragraph, from the article "cross ratio", articleId 28000.

<p>
 <assembly id="a62437" url="/assembly/view/62437">
  <media mediaid="7846" type="image" url="/46/7846-004-4FFD428F.jpg"/>
  <caption/>
  <credit>Encyclopædia Britannica, Inc.</credit>
 </assembly>
 <e type="bold">cross ratio</e>, in projective geometry, ratio
 that is of fundamental importance in determining projective transformations.
 In a projection of one line onto another from a central point
 (<e type="italic">see</e>
 <assemblyref assemblyid="a62437">Figure</assemblyref>), the
 double ratio of lengths on the first line (<e type="italic">AC</e>
 /<e type="italic">AD</e>)/(<e type="italic">BC</e>/
 <e type="italic">BD</e>) is equal to the corresponding ratio
 on the other line.
 [...]
</p>

Inline Media Reference: inlinemediaref

A reference to an inline image inlinemedia displayed elsewhere within the article.

XML Example

Two uses of inlinemediaref to refer to formulas shown elsewhere in the article, from "gas, state of matter", articleId 110302.

<h3 id="h51944">
 <title>Pressure</title>
 [...]
 <p>The energy law given as equation 
  (<inlinemediaref inlinemediaid="i15260">16</inlinemediaref>)
  also follows from equation
  (<inlinemediaref inlinemediaid="i15257">19</inlinemediaref>):
  the kinetic energy of translational motion per mole is (3/2)
  <e type="italic">R</e><e type="italic">T</e>.
  Any energy residing in the internal motions of the individual molecules
  is simply carried separately without contributing to the pressure.
 </p>
</h3>

Table Reference: tableref

A reference to a table found elsewhere in the article.

Text Formatting

Emphasis (Bold, Italic, Bold-Italic, Smallcaps, Underline): e

All text and font-style variants are captured by the emphasis em element. The type attribute specifies the exact emphasis style to be used.

XML Example

Two different type attribute values used with em in the article "North American Indian languages", articleId 109814.

<h1 id="h75311">
 <title>Bibliography</title>
 <p>
  <e type="smallcap">J.W. Powell</e>, “Indian Linguistic Families of
  America North of Mexico,” <e type="italic">U.S. Bureau of American
  Ethnology, 7th Annual Report</e> (1891), pp. 1–142, the first
  comprehensive classification; <e type="smallcap">Franz Boas</e>,
  <e type="italic">Handbook of American Indian Languages</e>,
  3 pt. (1911, 1922, and 1933–38), a classic introduction, with sketches of
  sample languages; <e type="smallcap">Harry Hoijer</e>
  <e type="italic">et al., Linguistic Structures of Native America</e>
  (1946), a summary of work on language classification and sketches
  of languages; [...]
 </p>
 [...]
</h1>

Superscript: sup

Text to be displayed in superscript.

XML Example

Examples of sup in the article "Xg blood group system", articleId 100538. Note the second instance contains an embedded formatting element.

<p>The group consists of one identifiable antigen, Xg<sup>a</sup>;
two phenotypes, Xg(a+) and Xg(a−); and a pair of alleles,
<e type="italic">Xg</e><sup><e type="italic">a</e></sup>,
which is dominant to <e type="italic">Xg</e>. This blood group 
follows the pattern for <xref articleid="66991">sex-chromosome</xref>
inheritance: daughters may receive a gene for Xg<sup>a</sup> from either the
mother or the father, but sons may only inherit Xg<sup>a</sup> from the mother. [...]</p>

Subscript: sub

Text to be displayed in subscript.

XML Example

Numerous uses of sub in the article "star", articleId 110472.

<p>The scale of magnitudes comprises a geometric progression of brightness.
 Magnitudes can be converted to light ratios by letting <e type="italic">l</e>
 <sub><e type="italic">n</e></sub> and <e type="italic">l</e>
 <sub><e type="italic">m</e></sub> be the brightnesses of stars of
 magnitudes <e type="italic">n</e> and <e type="italic">m</e>;
 the <xref articleid="48759">logarithm</xref> of the ratio of
 the two brightnesses then equals 0.4 times the difference between
 them—i.e., log(<e type="italic">l</e>
 <sub><e type="italic">m</e></sub>/<e type="italic">l</e>
 <sub><e type="italic">n</e></sub>)
 = 0.4(<e type="italic">n</e> − <e type="italic">m</e>).
 [...]
</p>

Overbar: overbar

An overbar, also known as bar, overline, or vinculum, is a horizontal line drawn on top of other characters to convey specialized meanings. For instance, in mathematics overbar identifies AB as a line segment, while in 1.1 it indicates a repeated decimal.

XML Example

Examples of overbar in the article "mineral", articleId 109683.

<p>A rotoinversion axis combines rotation about an axis of rotation
with inversion. Rotoinversion axes are symbolized as <overbar>1</overbar>,
<overbar>2</overbar>, <overbar>3</overbar>, <overbar>4</overbar>,
and <overbar>6</overbar>, where <overbar>1</overbar> is equivalent
to a centre of symmetry (or inversion), <overbar>2</overbar> is
equivalent to a mirror plane, and <overbar>3</overbar> is equivalent
to a threefold rotation axis plus a centre of symmetry. When the axis
of the crystal is vertical, <overbar>4</overbar> is characterized
by two top faces with identical faces upside down underneath. [...]</p>

Formula: formula

A mathematical or scientific formula meant to be presented as a separate design element on a page.

The formula element may also be used for constructs that are not formulas, strictly speaking, but similarly benefit from being set off from surrounding text (see for example use of formula with scanned poetry in the entry "prosody", articleId 110447).

XML Example

A formula element in the entry for "nuclear fusion", articleId 110415.

The binding energy <e type="italic">B</e> is the energy associated with
the mass difference between the <e type="italic">Z</e> protons and
<e type="italic">N</e> neutrons considered separately and the nucleons
bound together (<e type="italic">Z</e> + <e type="italic">N</e>) in a
nucleus of mass <e type="italic">M</e>. The formula is
 <formula>
  <e type="italic">B</e> = (<e type="italic">Z</e>
  <e type="italic">m</e><sub><e type="italic">p</e></sub>
  + <e type="italic">N</e><e type="italic">m</e>
  <sub><e type="italic">n</e></sub>
  − <e type="italic">M</e>)<e type="italic">c</e><sup>2</sup>,
 </formula>
where <e type="italic">m</e><sub><e type="italic">p</e></sub> and
<e type="italic">m</e><sub><e type="italic">n</e></sub> are the proton
and neutron masses and <e type="italic">c</e> is the speed of light.

Line Break: linebreak

Used to force a line break within a formula or line of poetry.

XML Example

Example (1): multiple mid-line linebreak elements within the poem "The Raven" by Edgar Allen Poe in the intermediate-level (ie, articleTypeId "31") article "poetry", articleId 276478. Note that mid-line line breaks are typically used to format poems with very long lines.

<line>But the Raven still beguiling my sad fancy into smiling,</line>
<line>Straight I wheeled a cushioned seat in front of bird and<linebreak/>
 bust and door;</line>
<line>Then, upon the velvet sinking, I betook myself to<linebreak/>
 linking</line>
<line>Fancy unto fancy, thinking what this ominous<linebreak/>
 bird of yore— —</line>
<line>What this grim, ungainly, ghastly, gaunt, and ominous<linebreak/>
 bird of yore</line>
<line>Meant in croaking “Nevermore.”</line>

Example (2): a forced linebreak within a formula in the article "analysis", articleId 111001.

<p>In order to specify physically realistic solutions, d’Alembert’s wave
equation must be supplemented by boundary conditions, which express the
fact that the ends of a violin string are fixed. Here the boundary
conditions take the form 
 <formula>
  <e type="italic">y</e>(0, <e type="italic">t</e>) = 0 and
  <linebreak/>
  <e type="italic">y</e>(<e type="italic">l</e>, <e type="italic">t</e>)
  = 0 for all <e type="italic">t</e>. (10)
 </formula>
[...]</p>

Fractions: fraction, num, den

A set of elements used to express a fraction and its numerator and denominator.

XML Example

Fractions whose numerators and denominators contain embedded formatting elements in the article "mathematics", articleId 109827.

An ellipsoid is a closed surface such that its 
intersection with any plane will produce an ellipse
or a circle. The formula for an ellipsoid is 
<fraction>
 <num><e type="italic">x</e><sup>2</sup></num>
 <den><e type="italic">a</e><sup>2</sup></den>
</fraction>
+ <fraction>
 <num><e type="italic">y</e><sup>2</sup></num>
 <den><e type="italic">b</e><sup>2</sup></den>
</fraction>
+ <fraction>
 <num><e type="italic">z</e><sup>2</sup></num>
 <den><e type="italic">c</e><sup>2</sup></den>
</fraction> = 1.

Integrals: integral, lower, upper

A set of elements used for notation of the integral symbol within an integral expression along with the lower and upper limits, or interval, of integration.

<integral>
  <lower><e type="italic">a</e></lower>
  <upper><e type="italic">b</e></upper>
</integral>
<e type="italic">f</e>(<e type="italic">t</e>)
<e type="italic">d</e><e type="italic">t</e>.

XML Example

An example of integral with upper and lower children in the article "analysis", articleId 111001.

The area of the region enclosed by the graph of the
 function <e type="italic">y</e> = 3<e type="italic">t</e>
 <sup>2</sup>, the horizontal axis, and the vertical lines
 <e type="italic">t</e> = 1 and <e type="italic">t</e> = 2,
 for example, is given by the integral 
 <integral>
  <lower>1</lower>
  <upper>2</upper>
 </integral>
 3<e type="italic">t</e><sup>2</sup>
 <e type="italic">d</e><e type="italic">t</e>.

Square Root: sqroot

An element that indicates a value's square root, or at times its cube or other root. A square root sign (also known as a root symbol, radical sign, or radix) should be displayed along with the value contained in sqroot.

XML Example

Three different types of square roots in the article "root", articleId 64056.

If <e type="italic">a</e> is a positive real number and
<e type="italic">n</e> a positive integer, there exists
a unique positive real number <e type="italic">x</e> such
that <e type="italic">x</e><sup><e type="italic">n</e></sup> =
<e type="italic">a</e>. This number—the (principal)
<e type="italic">n</e>th root of <e type="italic">a</e>—is written
 <sup>n</sup><sqroot> a </sqroot>
or <e type="italic">a</e><sup>1/<e type="italic">n</e></sup>.
The integer <e type="italic">n</e> is called the index of the root.
For <e type="italic">n</e> = 2, the root is called
the square root and is written
 <sqroot> <e type="italic">a</e> </sqroot>.
The root
 <sup>3</sup><sqroot> <e type="italic">a</e> </sqroot>
is called the cube root of <e type="italic">a</e>.