Wikipedia:Citing sources

Short and full citations

• A full citation fully identifies a reliable source and, where applicable, the place in that source (such as a page number) where the information in question can be found. For example: Rawls, John. A Theory of Justice. Harvard University Press, 1971, p. 1. This type of citation is usually given as a footnote, and is the most commonly used citation method in Wikipedia articles.
• A short citation is an inline citation that identifies the place in a source where specific information can be found, but without giving full details of the source. Some Wikipedia articles use it, giving summary information about the source together with a page number. For example, <ref>Rawls 1971, p. 1.</ref>, which renders as Rawls 1971, p. 1.. These are used together with full citations, which are listed in a separate "References" section, or have been provided in a footnote appearing earlier in the article.

Forms of short citations used include author–date referencing (APA style, Harvard style, or Chicago style), and author–title or author–page referencing (MLA style or Chicago style). As before, the list of footnotes is automatically generated in a "Notes" or "Footnotes" section, which immediately precedes the "References" section containing the full citations to the source. Short citations can be written manually, or by using either the {{sfn}} or {{harvnb}} templates or the {{r}} referencing template. The short citations and full citations may be linked so that the reader can click on the short note to find full information about the source. See the template documentation for details and solutions to common problems. For variations with and without templates, see wikilinks to full references. For a set of realistic examples, see these.

This is how short citations look in the edit box:

The Sun is pretty big,<ref>Miller 2005, p. 23.</ref> but the Moon is not so big.<ref>Brown 2006, p. 46.</ref> The Sun is also quite hot.<ref>Miller 2005, p. 34.</ref>

== Notes ==
<references />

== References ==
* Brown, Rebecca (2006). "Size of the Moon", ''Scientific American'', 51 (78).
* Miller, Edward (2005). ''The Sun''. Academic Press.

This is how they look in the article:

The Sun is pretty big,^[1] but the Moon is not so big.^[2] The Sun is also quite hot.^[3]

Notes

1. ^ Miller 2005, p. 23.
2. ^ Brown 2006, p. 46.
3. ^ Miller 2005, p. 34.

References

• Brown, Rebecca (2006). "Size of the Moon", Scientific American, 51 (78).
• Miller, Edward (2005). The Sun. Academic Press.

Shortened notes using titles rather than publication dates would look like this in the article:

Notes

1. ^ Miller, The Sun, p. 23.
2. ^ Brown, "Size of the Moon", p. 46.
3. ^ Miller, The Sun, p. 34.

When using manual links it is easy to introduce errors such as duplicate anchors and unused references. The script User:Trappist the monk/HarvErrors will show many related errors. Duplicate anchors may be found by using the W3C Markup Validation Service.

Multimedia

For an image or other media file, details of its origin and copyright status should appear on its file page. Image captions should be referenced as appropriate just like any other part of the article. A citation is not needed for descriptions such as alt text that are verifiable directly from the image itself, or for text that merely identifies a source (e.g., the caption "Belshazzar's Feast (1635)" for File:Rembrandt-Belsazar.jpg).

When not to cite

Citations are not used on disambiguation pages (since sourcing for the information given there should be done in the target articles).

Citations are often omitted from the lead section of an article, insofar as the lead summarizes information for which sources are given later in the article, although quotations and controversial statements, particularly if about living persons, should be supported by citations even in the lead. See WP:LEADCITE for more information.

Consecutive cites of the same source

Per MOS:CITEPUNCT, citations should be placed at the end of the text that they support. Material (e.g., the fact that elephants are mammals) that is repeated multiple times in a paragraph does not require an inline citation for every mention. If you say an elephant is a mammal more than once, provide one only at the first instance. Avoid cluttering text with redundant citations for the same facts, like this:

Elephants are large^[1] land^[2] mammals^[3] ... Elephants' teeth^[4] are very different^[4] from those of most other mammals.^[3][4] Unlike most mammals,^[3] which grow baby teeth^[5] and then replace them with a permanent set of adult teeth,^[4] elephants have cycles of tooth^[5] rotation throughout their entire^[6] lives.^[4]

This does not apply to lists or tables, nor does it apply when multiple sources support different parts of a paragraph or passage. Citation requirements for WP:DYK may require a citation to be inserted (for the duration of the DYK listing) even within a passage completely cited to the same sources.^{[note 2]}

How to place an inline citation using ref tags

To create a footnote, use the <ref>...</ref> syntax at the appropriate place in the article text, for example:

• Justice is a human invention.<ref>Rawls, John. ''A Theory of Justice''. Harvard University Press, 1971, p. 1.</ref> It ...

which will be displayed as something like:

• Justice is a human invention.^[1] It ...

It will also be necessary to generate the list of footnotes (where the citation text is actually displayed); for this, see § How to create the list of citations.

As in the above example, citation markers are normally placed after adjacent punctuation such as periods (full stops) and commas. For exceptions, see the WP:Manual of Style § Punctuation and footnotes. Note also that no space is added before the citation marker. Citations should not be placed within, or on the same line as, section headings.

The citation should be added close to the material it supports, offering text–source integrity. If a word or phrase is particularly contentious, an inline citation may be added next to that word or phrase within the sentence, but it is usually sufficient to add the citation to the end of the clause, sentence, or paragraph, so long as it's clear which source supports which part of the text.

What information to include

Listed below is the information that a typical inline citation or general reference will provide, though other details may be added as necessary. This information is included in order to identify the source, assist readers in finding it, and (in the case of inline citations) indicate the place in the source where the information is to be found. (If an article uses short citations, then the inline citations will refer to this information in abbreviated form, as described in the relevant sections above.)

In general, the citation information should be cited as it appears in the original source; exceptions are noted at MOS:CONFORMTITLE, MOS:TMRULES, MOS:NUMERO, and MOS:TE. For example, the album notes from Hurts 2B Human should not be cited as being from the album Hurts to be Human, or an X (formerly Twitter) user named "i😍dogs" should not be cited as "i[love]dogs". Do not use {{sic}}, per WP:QUOTETYPO.

Examples

Identifying parts of a source

When citing lengthy sources, you should identify which part of a source is being cited.

Links and ID numbers

A citation ideally includes a link or ID number to help editors locate the source. If you have a URL (web page) link, you can add it to the title part of the citation, so that when you add the citation to Wikipedia the URL becomes hidden and the title becomes clickable. To do this, enclose the URL and the title in square brackets—the URL first, then a space, then the title. For example:

''[https://monographs.iarc.fr/ENG/Monographs/vol66/mono66-7.pdf IARC Monographs On The Evaluation Of Carcinogenic Risks To Humans – Doxefazepam]''. International Agency For Research On Cancer (IARC). 66: 97–104. 13–20 February 1996.

For web-only sources with no publication date, the "Retrieved" date (or the date you accessed the web page) should be included, in case the web page changes in the future. For example: Retrieved 15 July 2011 or you can use the access-date parameter in the automatic Wikipedia:refToolbar 2.0 editing window feature.

You can also add an ID number to the end of a citation. The ID number might be an ISBN for a book, a DOI (digital object identifier) for an article or some e-books, or any of several ID numbers that are specific to particular article databases, such as a PMID number for articles on PubMed.

If your source is not available online, it should be available in reputable libraries, archives, or collections. If a citation without an external link is challenged as unavailable, any of the following is sufficient to show the material to be reasonably available (though not necessarily reliable): providing an ISBN or OCLC number; linking to an established Wikipedia article about the source (the work, its author, or its publisher); or directly quoting the material on the talk page, briefly and in context.

{{cite book |last=Rawls |first=John |date=1971 |title=A Theory of Justice |url=https://books.google.com/books?id=kvpby7HtAe0C&pg=PA18 |publisher=Harvard University Press |page=18 |via=Google Books}}

Rawls, John. [https://books.google.com/books?id=kvpby7HtAe0C&pg=PA18 ''A Theory of Justice'']. Harvard University Press, 1971, p. 18.

Say where you read it

"Say where you read it" follows the practice in academic writing of citing sources directly only if you have read the source yourself. If your knowledge of the source is secondhand—that is, if you have read Jones (2010), who cited Smith (2009), and you want to use what Smith (2009) said—make clear that your knowledge of Smith is based on your reading of Jones.

When citing the source, write the following (this formatting is just an example):

John Smith (2009). Name of Book I Haven't Seen, Cambridge University Press, p. 99, cited in Paul Jones (2010). Name of Encyclopedia I Have Seen, Oxford University Press, p. 29.

Or if you are using short citations:

Smith (2009), p. 99, cited in Jones (2010), p. 29.

The same principle applies when indicating the source of images and other media files in an article.

Note: The advice to "say where you read it" does not mean that you have to give credit to any search engines, websites, libraries, library catalogs, archives, subscription services, bibliographies, or other sources that led you to Smith's book. If you have read a book or article yourself, that's all you have to cite. You do not have to specify how you obtained and read it.

So long as you are confident that you read a true and accurate copy, it does not matter whether you read the material using an online service like Google Books; using preview options at a bookseller's website like Amazon; through your library; via online paid databases of scanned publications, such as JSTOR; using reading machines; on an e-reader (except to the extent that this affects page numbering); or any other method.

Dates and reprints

Date a book that is identically reprinted or printed-on-demand to the first date in which the edition became available. For example, if an edition of a book was first released in 2005 with an identical reprinting in 2007, date it to 2005. If substantive changes were made in a reprint, sometimes marked on the verso with "Reprinted with corrections", note the edition and append the corrected reprint year to it (e.g. "1st ed. reprinted with corrections 2005").

Editors should be aware that older sources (especially those in the public domain) are sometimes republished with modern publication dates; treat these as new publications. When this occurs and the citation style being used requires it, cite both the new and original publication dates, e.g.:

• Darwin, Charles (1964) [1859]. On the Origin of Species (facsimile of 1st ed.). Harvard University Press.

This is done automatically in the {{citation}} and {{cite book}} templates when you use the |orig-date= parameter.

Alternatively, information about the reprint can be appended as a textual note:

• Boole, George (1854). An Investigation of the Laws of Thought on Which Are Founded the Mathematical Theories of Logic and Probabilities. Macmillan. Reprinted with corrections, Dover Publications, New York, NY, 1958.

Seasonal publication dates and differing calendar systems

Publication dates, for both older and recent sources, should be written with the goal of helping the reader find the publication and, once found, confirm that the correct publication has been located. For example, if the publication date bears a date in the Julian calendar, it should not be converted to the Gregorian calendar.

If the publication date was given as a season or holiday, such as "Winter" or "Christmas" of a particular year or two-year span, it should not be converted to a month or date, such as July–August or December 25. If a publication provided both seasonal and specific dates, prefer the specific one.

Additional annotation

In most cases it is sufficient for a citation footnote simply to identify the source (as described in the sections above); readers can then consult the source to see how it supports the information in the article. Sometimes, however, it is useful to include additional annotation in the footnote, for example to indicate precisely which information the source is supporting (particularly when a single footnote lists more than one source – see § Bundling citations and § Text–source integrity, below).

A footnote may also contain a relevant quotation from the source. This is especially helpful when the cited text is long or dense. A quotation allows readers to immediately identify the applicable portion of the reference. Quotes are also useful if the source is not easily accessible. However, caution should be exercised, as always, to avoid copyright violations.

In the case of non-English sources, it may be helpful to quote from the original text and then give an English translation. If the article itself contains a translation of a quote from such a source (without the original), then the original should be included in the footnote. (See the WP:Verifiability § Non-English sources policy for more information.)

General references

A general reference is a citation to a reliable source that supports content, but is not linked to any particular text in the article through an inline citation. General references are usually listed at the end of the article in a "References" section, and are usually sorted by the last name of the author or the editor. General reference sections are most likely to be found in underdeveloped articles, especially when all article content is supported by a single source. The disadvantage of general references is that text–source integrity is lost, unless the article is very short. They are frequently reworked by later editors into inline citations.

The appearance of a general references section is the same as those given above in the sections on short citations and parenthetical references. If both cited and uncited references exist, their distinction can be highlighted with separate section names, e.g., "References" and "General references".

How to create the list of citations

With some exceptions discussed below, citations appear in a single section containing only the <references /> tag or the {{Reflist}} template. For example:

== References ==
{{Reflist}}

or

== References ==
<references />

The footnotes will then automatically be listed under that section heading. Each numbered footnote marker in the text is a clickable link to the corresponding footnote, and each footnote contains a caret that links back to the corresponding point in the text.

Scrolling lists, or lists of citations appearing within a scroll box, should never be used. This is because of issues with readability, browser compatibility, accessibility, printing, and site mirroring.^{[note 4]}

If an article contains a list of general references, this is usually placed in a separate section, titled, for example, "References". This usually comes immediately after the section(s) listing footnotes, if any. (If the general references section is called "References", then the citations section is usually called "Notes".)

Citations in explanatory footnotes

Explanatory footnotes require citations, like all content. When an article contains both footnoted citations and other (explanatory) footnotes, it is possible (but not necessary) to divide them into two separate lists using footnotes groups. Explanatory footnotes and citations are then placed in separate sections, called (for example) "Notes" and "References", respectively.

They may be written using citations in footnotes (this requires templates such as {{efn}} or {{refn}}).

The Sun is very big.^[1][A]

Notes

1. ↑ The moon is also very big.^[2]

References

1. ↑ Miller (2002). Size of the Sun.
2. ↑ Hsu (2003). Size of the Moon.

Explanatory notes and citations can also be mixed, possibly including short citations referring to earlier footnotes or to a section (often called "Bibliography" or similar) where full citation details can be found.

The Sun is very big.^[1][2]

References

1. ↑ Miller 2008.
2. ↑ The moon is also very big (Hsu 2007).

Bibliography

• Hsu (2007). Size of the Moon.
• Miller (2008). Size of the Sun.

Duplicate citations

Combine precisely duplicated full citations. In this context, "precisely duplicated" means having the same content, not necessarily identical strings ("The New York Times" is the same as "NY Times"; different access-dates are not significant). Do not discourage editors, particularly inexperienced ones, from adding duplicate citations when the use of the source is appropriate, because a duplicate is better than no citation. But any editor should feel free to combine them, and doing so is the best practice on Wikipedia.

Citations to different pages or parts of the same source can also be combined (preserving the distinct parts of the citations), as described in Help:References and page numbers. Any method that is consistent with the existing citation style (if any) may be used, or consensus can be sought to change the existing style. Some tools are listed at Help:Citation tools § Duplicate reference finders.

Variation in citation methods

Editors should not attempt to change an article's established citation style, merely on the grounds of personal preference or to make it match other articles, without first seeking consensus for the change.^{[note 6]}

As with spelling differences, it is normal practice to defer to the style used by the first major contributor or adopted by the consensus of editors already working on the page, unless a change in consensus has been achieved. If the article you are editing is already using a particular citation style, you should follow it; if you believe it is inappropriate for the needs of the article, seek consensus for a change on the talk page. If you are the first contributor to add citations to an article, you may choose whichever style you think best for the article, except inline parenthetical referencing.

An article where all or most of the citations fail to provide needed bibliographic data does not have a consistent citation style, and can be changed freely to insert such data. Such data includes the name of the source, the title of the article or web page consulted, the author (if known), the publication date (if known), and the page numbers (where relevant). The data provided should be sufficient to uniquely identify the source, allow readers to find it, and allow readers to initially evaluate a source without retrieving it.

Parenthetical referencing

"WP:PARENTHETICAL" redirects here. For guidelines on the use of parentheses (round brackets), see WP:Manual of Style § Brackets and parentheses.

Shortcut

• WP:PARENWP:PAREN

Inline parenthetical referencing is deprecated on Wikipedia.^{[note 7]} This includes short citations in parentheses placed within the article text itself, such as (Smith 2010, p. 1). This does not affect short citations that use <ref> tags, which are not inline parenthetical references; see the section on short citations above for that method. This also does not affect explanatory footnotes. Some editors prefer to minimize the number of layers of footnoting for readability, but some featured articles have three layers of footnote (explanatory note, short cite, and full cite).

This should no longer be used, and should be replaced with footnotes if encountered:

N

The Sun is pretty big (Miller 2005, p. 1), but the Moon is not so big (Brown 2006, p. 2). The Sun is also quite hot (Miller 2005, p. 3).

References

• Brown, R. (2006). "Size of the Moon", Scientific American, 51 (78).
• Miller, E. (2005). The Sun, Academic Press.

Avoid embedded links

Embedded links to external websites should not be used as inline citations, because they are highly susceptible to link rot. That is, links like:[example.com], which is rendered as: , should not be used as a citation. Avoid raw links even if placed between ref tags (e.g. <ref>[example.com]</ref>). Since any citation that supports the preceding text is better than none, do not revert the good-faith addition of partial citations. They should be replaced with more complete, properly formatted citations as soon as possible.

Embedded links should never be used to place external links in the content of an article, like this: "Example Inc. announced their latest product ...".

Convenience links

A convenience link is a link to a copy of your source on a web page provided by someone other than the original publisher or author. For example, a copy of a newspaper article no longer available on the newspaper's website may be hosted elsewhere. When offering convenience links, it is important to be reasonably certain that the convenience copy is a true copy of the original, without any changes or inappropriate commentary, and that it does not infringe the original rights-holders' copyrights. Accuracy can be assumed when the hosting website appears reliable.

For academic sources, the convenience link is typically a reprint provided by an open-access repository, such as the author's university's library or institutional repository. Such green open access links are generally preferable to paywalled or otherwise commercial and unfree sources.

Where several sites host a copy of the material, the site selected as the convenience link should be the one whose general content appears most in line with Wikipedia:Neutral point of view and Wikipedia:Verifiability.

Indicating availability

If your source is not available online, it should be available in libraries, archives, or collections. If a citation without an external link is challenged as unavailable, any of the following is sufficient to show the material to be reasonably available (though not necessarily reliable): providing an ISBN or OCLC number; linking to an established Wikipedia article about the source (the work, its author, or its publisher); or directly quoting the material on the talk page, briefly and in context.

Links to sources

For a source available in hardcopy, microform, and/or online, omit, in most cases, which one you read. While it is useful to cite author, title, edition (1st, 2nd, etc.), and similar information, it generally is not important to cite a database such as ProQuest, EBSCOhost, or JSTOR (see the list of academic databases and search engines) or to link to such a database requiring a subscription or a third party's login. The basic bibliographic information you provide should be enough to search for the source in any of these databases that have the source. However, you may provide the DOI, ISBN, or another uniform identifier, if available. If the publisher offers a link to the source or its abstract that does not require a payment or a third party's login for access, you may provide the URL for that link. If the source only exists online, give the link even if access is restricted (see WP:PAYWALL).

Preventing and repairing dead links

To help prevent dead links, persistent identifiers are available for some sources. Some journal articles have a digital object identifier (DOI); some online newspapers and blogs, and also Wikipedia, have permalinks that are stable. When permanent links aren't available, consider making an archived copy of the cited document when writing the article; on-demand web archiving services such as the Wayback Machine (https://web.archive.org/save) are fairly easy to use (see pre-emptive archiving).

Do not delete a citation merely because the URL is not working. Dead links should be repaired or replaced if possible. If you encounter a dead URL being used as a reliable source to support article content, follow these steps prior to deleting it:

1. Confirm status: First, check the link to confirm that it is dead and not temporarily down. The online service "Is it down right now?" can help to determine if a site is down.
2. Check for a changed URL on the same website: Pages are frequently moved to different locations on the same site. The site's error page may have a "Search" box; alternatively, in both the Google and DuckDuckGo search engines – among others – the keyterm "site:" can be used. For instance: site:nytimes.com "the goose is loose".
3. Check for web archives: If a web page is unavailable, check whether an archived copy exists. Common web archives on Wikipedia include:
   • The Internet Archive has over a trillion archived web pages. See Wikipedia:Using the Wayback Machine.
   • The UK Government Web Archive (https://www.nationalarchives.gov.uk/webarchive/) preserves 1500 UK central government websites.
   If multiple archive dates are available, try to use one that is most likely to contain the page contents as of the |access-date=. If that parameter is not specified, search the article's revision history to determine when the link was added to the article.
   For most citation templates, archive locations are entered using the |archive-url=, |archive-date= and |url-status= parameters. The primary link is switched to the archive link when |url-status=dead. This retains the original link location for reference. If the web page now leads to a completely different website, set |url-status=usurped to hide the original website link in the citation.
   Note that some archives operate with a delay. Wait before concluding that no archive exists. Dead URLs to reliable sources should normally be tagged with {{dead link|date={{subst:CURRENTMONTHNAME}} {{subst:CURRENTYEAR}}}}, so that you can estimate how long the link has been dead.
   Bookmarklets to check common archive sites for archives of the current page:

   Archive.org

   javascript:void(window.open('https://web.archive.org/web/*/'+location.href))

   Mementos interface

   javascript:void(window.open('https://www.webarchive.org.uk/mementos/search/'+encodeURIComponent(location.href)+'?referrer='+encodeURIComponent(document.referrer)))

4. Remove dead convenience links: If the material was published on paper (e.g., academic journal, newspaper article, magazine, book), then the dead URL is not necessary. Simply remove the dead URL, leaving the remainder of the reference intact.
5. Find a replacement source: Search the web for quoted text, the article title, and parts of the URL. Consider contacting the website/person that originally published the reference and asking them to republish it. Ask other editors for help finding the reference somewhere else, including the user who added the reference. Find a different source that says essentially the same thing as the reference in question.
6. Remove hopelessly-lost web-only sources: If the source material does not exist offline, and if there is no archived version of the web page (be sure to wait ~24 months), and if you cannot find another copy of the material, then the dead citation should be removed and the material it supports should be regarded as unverified if there is no other supporting citation. If it is material that is specifically required by policy to have an inline citation, then please consider tagging it with {{citation needed}}. It may be appropriate for you to move the citation to the talk page with an explanation, and notify the editor who added the now-dead link.

Keeping citations close

Editors should exercise caution when rearranging or inserting material to ensure that text–source relationships are maintained. References should not be moved if doing so might break the text–source relationship.

When new text is inserted into a paragraph, make sure it is supported by the existing source or a new source. If a sentence or paragraph is footnoted with a source, adding new material that is not supported by the existing source to it, without a source for the new text, is highly misleading if placed so as to appear that the cited source supports it. For example, when editing text originally reading

The Sun is pretty big.^[1]

Notes

---

1. ^ Miller, Edward. The Sun. Academic Press, 2005, p. 1.

an edit that does not imply that the new material is supported by the same reference is

The Sun is pretty big.^[1] The Sun is also quite hot.^[2]

Notes

---

1. ^ Miller, Edward. The Sun. Academic Press, 2005, p. 1.
2. ^ Smith, John. The Sun's Heat. Academic Press, 2005, p. 2.

Do not add other facts or assertions into a fully cited paragraph or sentence:

N

The Sun is pretty big, but the Moon is not so big.^[1] The Sun is also quite hot.^[2]

Notes

---

1. ^ Miller, Edward. The Sun. Academic Press, 2005, p. 1.
2. ^ Smith, John. The Sun's Heat. Academic Press, 2005, p. 2.

Include a source to support the new information. There are several ways to write this, including:

Y

The Sun is pretty big,^[1] but the Moon is not so big.^[2] The Sun is also quite hot.^[3]

Notes

---

1. ^ Miller, Edward. The Sun. Academic Press, 2005, p. 1.
2. ^ Brown, Rebecca. "Size of the Moon", Scientific American, 51 (78): 46.
3. ^ Smith, John. The Sun's Heat. Academic Press, 2005, p. 2.

Citation order

There is no consensus for a specific ordering of citations, and editors should not edit-war over it, or make mass changes of ordering to suit personal preferences. In particular, references need not be moved solely to maintain the numerical order of footnotes as they appear in the article.

Bundling citations

Sometimes the article is more readable if multiple citations are bundled into a single footnote. For example, when there are multiple sources for a given sentence, and each source applies to the entire sentence, the sources can be placed at the end of the sentence, like this:^[4][5][6][7] Or they can be bundled into one footnote at the end of the sentence or paragraph, like this:^[4]

Bundling is also useful if the sources each support a different portion of the preceding text, or if the sources all support the same text. Bundling has several advantages:

• It avoids the visual clutter of multiple clickable footnotes inside a sentence or paragraph;
• It makes it less likely that inline citations will be moved inadvertently when text is re-arranged, because the footnote states clearly which source supports which point.

Keep in mind that there can be some disadvantages as well:

• When multiple sources are bundled into a single citation, it can be unclear which source supports which specific point, particularly if the text contains multiple claims.
• Readers and editors may need to cross-check all sources in the bundle to verify a single point, increasing the time and effort required for fact-checking.
• Bundling can inadvertently suggest that all sources equally support all points in the cited text, which may not always be accurate.
• Rearranging or modifying text later can create confusion or errors if the bundled note needs to be updated or split to reflect new changes.
• If an additional source is required for one use of the bundle, then adding it to the bundle will add a useless reference to all the others, or else require a new bundle duplicating all the other references, defeating the purpose of bundling.

To concatenate multiple citations for the same content into a single footnote, there are several layouts available, as illustrated below:

Simply using line breaks to separate list items breaches MOS:Accessibility § Nobreaks: "<br /> line breaks ... should not be used." {{Unbulleted list citebundle}} a.k.a. {{Multiref}} was made specifically for this purpose. Some other templates in the same vein are listed at the disambiguation page Template:Multiple references.

Within a given article only a single layout should generally be used, except that inline may always be appropriate for shortened references, often all for the same statement:

Metadata

Citations may be accompanied by metadata, though it is not mandatory. Most citation templates on Wikipedia use the COinS standard. Metadata such as this allow browser plugins and other automated software to make citation data accessible to the user, for instance by providing links to their library's online copies of the cited works. In articles that format citations manually, metadata may be added manually in a span, according to the COinS specification.