Citation machine

Broadly, a citation is a reference to a published or unpublished source (not always the original source). More precisely, a citation is an abbreviated alphanumeric expression  embedded in the body of an intellectual work that denotes an entry in the bibliographic references section of the work for the purpose of acknowledging the relevance of the works of others to the topic of discussion at the spot where the citation appears. Generally the combination of both the in-body citation and the bibliographic entry constitutes what is commonly thought of as a citation (whereas bibliographic entries by themselves are not).

A prime purpose of a citation is intellectual honesty: to attribute prior or unoriginal work and ideas to the correct sources, and to allow the reader to determine independently whether the referenced material supports the author's argument in the claimed way.
The forms of citations generally subscribe to one of the generally accepted citations systems, such as the Oxford, Harvard, MLA, American Sociological Association (ASA), American Psychological Association (APA), and other citations systems, as their syntactic conventions are widely known and easily interpreted by readers. Each of these citation systems has its respective advantages and disadvantages relative to the trade-offs of being informative (but not too disruptive) and thus should be chosen relative to the needs of the type of publication being crafted. Editors will often specify the citation system to use.
Bibliographies, and other list-like compilations of references, are generally not considered citations because they do not fulfill the true spirit of the term: deliberate acknowledgment by other authors of the priority of one's ideas.


Citation content can vary depending on the type of source and may include:
Book: author(s), book title, publisher, date of publication, and page number(s) if appropriate.

Journal: author(s), article title, journal title, date of publication, and page number(s).
Newspaper: author(s), article title, name of newspaper, section title and page number(s) if desired, date of publication.
Web site: author(s), article and publication title where appropriate, as well as a URL, and a date when the site was accessed.

Play: inline citations offer part, scene, and line numbers, the latter separated by periods: 4.452 refers to scene 4, line 452. For example, "In Eugene Onegin, Onegin rejects Tanya when she is free to be his, and only decides he wants her when she is already married" (Pushkin 4.452-53).
Poem: spaced slashes are normally used to indicate separate lines of a poem, and parenthetical citations usually include the line number(s). For example: "For I must love because I live / And life in me is what you give." (Brennan, lines 15–16).

Note systems involve the use of sequential numbers in the text which refer to either footnotes (notes at the end of the page) or endnotes (a note on a separate page at the end of the paper) which gives the source detail. The notes system may or may not require a full bibliography, depending on whether the writer has used a full note form or a shortened note form.

For example, an excerpt from the text of a paper using a notes system without a full bibliography could look like this:
"The five stages of grief are denial, anger, bargaining, depression, and acceptance."1
The note, located either at the foot of the page (footnote) or at the end of the paper (endnote) would look like this:
1. Elisabeth Kübler-Ross, On Death and Dying (New York: Macmillan, 1969) 45–60.
In a paper which contains a full bibliography, the shortened note could look like this:
1. Kübler-Ross, On Death and Dying 45–60.
and the bibliography entry, which would be required with a shortened note, would look like this:
Kübler-Ross, Elisabeth. On Death and Dying. New York: Macmillan, 1969.
In the humanities, many authors use footnotes or endnotes to supply anecdotal information. In this way, what looks like a citation is actually supplementary material, or suggestions for further reading.

Social sciences
The style of the American Psychological Association, or APA style, published in the Publication Manual of the American Psychological Association, is most often used in social sciences. APA style uses Harvard referencing within the text, listing the author's name and year of publication, keyed to an alphabetical list of sources at the end of the paper on a References page.

The American Political Science Association publishes both a style manual and a style guide for publications in this field.The style is close to the CMOS.
The American Anthropological Association utilizes a modified form of the Chicago Style laid out in their Publishing Style Guide.
The ASA style of American Sociological Association is one of the main styles used in sociological publications.


Humanities
The Chicago Style (CMOS) was developed and its guide is The Chicago Manual of Style. It is most widely used in history and economics as well as some social sciences. Its derivative is the closely related Turabian style which is designed for student references and is distinguished from the CMOS by omission of quotation marks in reference lists and mandatory access date citation.
The Columbia Style was made by Janice R. Walker and Todd Taylor to give detailed guidelines for citing internet sources. Columbia Style offers models for both the humanities and the sciences.
Evidence Explained: Citing History Sources from Artifacts to Cyberspace by Elizabeth Shown Mills covers primary sources not included in CMOS, such as censuses, court, land, government, business, and church records. Includes sources in electronic format. Used by genealogists and historians.[13]
Harvard referencing (or author-date system) is a specific kind of parenthetical referencing. Parenthetical referencing is recommended by both the British Standards Institution and the Modern Language Association. Harvard referencing involves a short author-date reference, e.g., "(Smith, 2000)", being inserted after the cited text within parentheses and the full reference to the source being listed at the end of the article.
MLA style was developed by the Modern Language Association and is most often used in the arts and the humanities, particularly in English studies, other literary studies, including comparative literature and literary criticism in languages other than English ("foreign languages"), and some interdisciplinary studies, such as cultural studies, drama and theatre, film, and other media, including television. This style of citations and bibliographical format uses parenthetical referencing with author-page (Smith 395) or author-short title-page (Smith, Contingencies 42) in the case of more than one work by the same author within parentheses in the text, keyed to an alphabetical list of sources on a "Works Cited" page at the end of the paper, as well as notes (footnotes or endnotes). See The MLA Style Manual and The MLA Handbook for Writers of Research Papers, particularly Citation and bibliography format.
The MHRA Style Guide is published by the Modern Humanities Research Association (MHRA) and most widely used in the arts and humanities in the United Kingdom, where the MHRA is based. It is available for sale both in the UK and in the United States. It is similar to MLA style, but has some differences. For example, MHRA style uses footnotes that reference a citation fully while also providing a bibliography. Some readers find it advantageous that the footnotes provide full citations, instead of shortened references, so that they do not need to consult the bibliography while reading for the rest of the publication details.
In some areas of the Humanities, footnotes are used exclusively for references, and their use for conventional footnotes (explanations or examples) is avoided. In these areas, the term "footnote" is actually used as a synonym for "reference", and care must be taken by editors and typesetters to ensure that they understand how the term is being used by their authors.


Legal citation
The Bluebook is a citation system traditionally used in American academic legal writing, and the Bluebook (or similar systems derived from it) are used by many courts. At present, academic legal articles are always footnoted, but motions submitted to courts and court opinions traditionally use inline citations which are either separate sentences or separate clauses.
The legal citation style used almost universally in Canada is based on the Canadian Guide to Uniform Legal Citation (aka McGill Guide), published by McGill Law Journal.


Issues


In their research on footnotes in scholarly journals in the field of communication, Michael Bugeja and Daniela V. Dimitrova have found that citations to online sources have a rate of decay (as cited pages are taken down), which they call a "half-life," that renders footnotes in those journals less useful for scholarship over time.