A bibliographic description of a work (such as a journal article, book, or personal communication) used as a reference for an book or chapter or referred to in the text. Citations may be cited directly in the text of an article or listed in a bibliographic reference list at the end of the article with the reference list item cited from within the text. Citations are typically referenced from within the text of the work.


Although in this tag set, citations are allowed to float freely within a paragraph, most journals require that the full citation be listed in the bibliographic reference list <ref-list> and only a X(cross) Reference <xref> be inserted into the text at the place of reference.

Best Practice: Although the NCBI Book Tag Sets do not enforce it, best practice is to tag as many of the following elements as possible within a citation to a journal article or a book, so that PubMed Central, CrossRef, or other matching service can make the journal citation into a live link:


The title of a journal, book, conference proceedings, etc. that is the source of the cited material. (Note: In PubMed Central processing, this is typically the MEDLINE abbreviation of the journal name.)


Title of the article


Volume of the journal


Issue of the journal


Page number on which the article starts


Name of an author or editor


Year of publication


Month of publication (if present)


Date of publication (if present)

The other elements may be tagged if desired. Use the <source> element for titles of books, conference proceedings, etc.

Linking Note: In order to make citations into live links, as much of the author and date information as is available should be preserved, even if it not possible to tag all the element just named. The most important date tag is <year>, and it should always be tagged if possible, for example, <year>2003</year>. The <day> and <month> tags are used more rarely; they are provided because some of the citation matching services can use the month and day information if it is available.

Punctuation and Spacing: In those instances in which portions of a citation are not tagged, all punctuation and spacing must be included as part of the text in order to enable rendering for display. If a citation is fully tagged, there are two styles of handling punctuation and spacing: 1) all the periods, commas, spaces, and other punctuation may be in the tagged citation (preferably in the citation but outside the internal tagging in the citation (e.g., “<lpage>236</lpage>.) or 2) all spacing and punctuation may be completely removed.”

Incomplete References: Some bibliographic references lists identify successive bibliographic references by the same author or involving the same journal by omitting the duplicated portion of the reference and inserting a vertical rule or the word “Ibid” or “Id.” instead of the author’s name or the journal title. Since it is the intention of the NCBI Book Tag Set to preserve the information provided in a bibliographic reference, and since best practice tagging would make each bibliographic reference accessible for CrossRef (and similar) queries, such references should be enhanced by tagging the author’s name or the journal title based on the name or title provided in the proceeding reference. The word “Ibid” or “Id.” may also be retained as part of the textual content.


citation-type Type of Citation
id Identifier
xlink:actuate Actuating the Link
xlink:href Href (Linking Mechanism)
xlink:role Role of the Link
xlink:show Showing the Link
xlink:title Title of the Link
xlink:type Type of Link
xmlns:xlink XLink Namespace Declaration

Related Elements

There are two kinds of citations in this tag set; the <citation> element is free form to accommodate many styles of references, while the other element (<nlm-citation>) has a rigid structure of elements within it.

Model Information

Content Model

<!ELEMENT  citation     (#PCDATA %citation-elements;)*               >


Any combination of:

This element may be contained in:

<alt-title> Alternate Title; <article-title> Article Title; <book-title> Book Title; <collection-name> Collection Name; <p> Paragraph; <ref> Reference Item; <subtitle> Subtitle; <td> Table Data Cell (XHTML table model); <th> Table Header Cell (XHTML table model); <title> Title; <trans-subtitle> Translated Subtitle; <trans-title> Translated Title

Tagged Examples

Example 1

In a bibliographic reference (punctuation and spacing removed):

<ref id="bid.41">
<article-title>A common language for physical mapping 
of the human genome</article-title>
<pub-id pub-id-type="pmid">2781285</pub-id>

Example 2

In a bibliographic reference (punctuation and spacing preserved):

<ref id="B8"><label>8</label>
</name>. <article-title>Effects and costs of 
day-care services for the chronically ill: a randomized 
experiment</article-title>. <source>Medical Care</source>
<year>1980</year>; <volume>18</volume>:
<pub-id pub-id-type="publisher-id">WES-6772889</pub-id>.