Wikipedia:PAREN

Parenthetical referencing is a citation system in which in-text citations are made using parentheses. Parenthetical referencing may be used instead of footnotes or endnotes; or, in the case of some documentation systems, with the use of footnotes and/or endnotes, called "content notes".

There are two main styles of parenthetical referencing: the "author-date system", which is primarily used in the social sciences and exemplified in the official style manual and related style guides published by the American Psychological Association (APA), such as The Publication Manual of the American Psychological Association; and the "author-title system", which is primarily used in the arts and the humanities and exemplified in the MLA Style Manual and the MLA Handbook for Writers of Research Papers, both official publications of the Modern Language Association (MLA).

Both "author-date" and "author-title" systems are used in journal publications and in book publishing. Book publishers specify their publishing "house styles" for their authors, directing which documentation system to use. Many American book publishers, such as the University of Chicago Press and Duke University Press, for example, require documentation systems presented in The Chicago Manual of Style, which is flexible in that it offers writers a choice of several different formats, including both "author-date" and "author-title" parenthetical citation referencing methods, depending on which method the publisher and its editorial staff considers most appropriate to the content discipline(s).

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