Citation Systems

by Tim Gorichanaz, for Dr. Deborah Turner’s Academic Library Service course

Writers often reference the words and ideas of others. For a discussion on referencing in everyday life, see my “Referencing Issues in Everyday Life” lecture.

In academic writing, it is especially important to identify these references, which is accomplished with citation. We practice citation for many reasons:

There are many different systems that can be used to cite the work of others. In general, citation systems make use of abbreviated citations within the text that point the reader to the bibliographic references section, where the full details of the sources appear. The various citation systems differ in the exact formatting and detail provided. In general, though, the information in the citations includes:

Citation systems fall into two broad categories:

  1. Vancouver System: Sequential numbers (usually superscript) in the text that refer the reader to footnotes or endnotes for source details
  2. Parenthetical Referencing: Parenthesis-enclosed in-text citations that generally refer the reader to reference list for source detail (also called Harvard Referencing) 

The most common citation systems are American Psychological Association (APA), Modern Language Association (MLA), Chicago Manual of Style (CMOS/Chicago) and Turabian, which are outlined below. Different citation systems are conventionally favored by different fields of study, organizations and publications. The required citation system will be specified by the instructor or editor in each specific case.

It should be noted that these style guides comprise far more than just citation systems; they also offer advice and requirements in terms of word choice, sentence construction, spelling, punctuation, paper formatting and the like. Writing “in APA style,” for example, has just as much to do with one-inch margins and twelve-point Times New Roman as it does with including the year in your in-text citations.


A Few Common Styles

American Psychological Association (APA)

APA is mostly used in the social and behavioral sciences. The APA first developed its guidelines in 1929 with the stated goals of improving clarity and eliminating bias. The Publication Manual of the American Psychological Association was most recently revised in its sixth edition, which came out in 2010.

The social sciences place a high value on authorship and chronology, so that information is given within the text itself. APA uses parenthetical references that give the author’s last name and the year of publication, keyed to an alphabetical list of sources at the end of the document on the References page.

The in-text citations look like this (Pollan, 2006), and an example of the corresponding reference can be seen below:

Pollan, M. (2006). The omnivore’s dilemma: A natural history of four meals. New York: Penguin.

Learn more about this style at

For a good, free guide on documenting many different kinds of sources in APA style, see

Modern Language Association (MLA)

MLA style is most often used in the arts and the humanities, particularly in literary and modern language studies, as well as some interdisciplinary fields such as cultural and media studies. It was first developed in 1985 by the Modern Language Association. The latest guide on MLA style is the seventh-edition MLA Handbook for Writers of Research Papers (2009), which is primarily meant for undergraduate and high school students. The organization also publishes the more ample MLA Style Manual and Guide to Scholarly Publishing meant for graduate students and professional writers, the third edition of which came out in 2008.

MLA uses parenthetical references that give the author’s last name and the page number of the referenced information. The in-text citation leads readers to the corresponding entry on the Works Cited page, which is ordered alphabetically. Language studies place a high value on authorship; on the Works Cited page, the full name of the author is given, rather than just their last name and first initial.

The in-text citations look like this (Pollan 56), and an example of the corresponding full reference can be seen below:

Pollan, Michael. The Omnivore’s Dilemma: A Natural History of Four Meals. New York: Penguin, 2006. Print.

Learn more about this style at

For a good, free guide on documenting many different kinds of sources in MLA style, see

Chicago Manual of Style (CMOS/Chicago)

Chicago style is most often used in history and economics, as well as some social sciences. It was developed in 1906 to outline the editorial expectations of the University of Chicago Press. It was one of the first style guides in the United States, and it is largely responsible for the standardization of citation style in research. The sixteenth edition of the Chicago Manual of Style was published in 2010.

The Chicago Manual of Style allows for both Vancouver and Parenthetical referencing (CMOS calls these notes–bibliography style and author–date style, respectively). Choosing between the two depends on the conventions of each research group. Historians, for example, tend to favor the Vancouver style, placing their references in footnotes right on the page where the reference occurs. This is because historians often deal with esoteric and ancient works that aren’t as conducive to the author–date notation. This style also demonstrates a value for in-context referencing of complete bibliographic information, so that readers can see source details more easily than in other styles. It should be noted that, even with the notes–bibliography style, researchers often include a final bibliography that aggregates all references mentioned throughout the text.

With the notes–bibliography style, a reference will be noted with a superscript number like this1, and then the complete reference will be given in the footer of the page:

1. Michael Pollan, The Omnivore’s Dilemma: A Natural History of Four Meals (New York: Penguin, 2006), 56.

Author–date referencing, on the other hand, tends to be preferred by researchers in the sciences. With this style, in-text citations appear like this (Pollan 2006, 56), and readers then must flip to the bibliography to find the complete reference:

Pollan, Michael. 2006. The Omnivore’s Dilemma: A Natural History of Four Meals. New York: Penguin.

Learn more about this style at

For a good, free guide on documenting many different kinds of sources in CMOS, see

To see a detailed table comparing APA, MLA and CMOS, including at-a-glance comparison of many specific types of references, check out this PDF.


In 1937, the University of Chicago Press published A Manual for Writers of Research Papers, Theses, and Dissertations, written by Kate Turabian. This book presented what has come to be known as Turabian style, which is primarily meant for students. The eighth edition of the manual was published in 2013.

Turabian style is almost the same as Chicago style. Most notably, it omits quotation marks in reference lists and adds the date of access of the source.


This has been an overview of some of the major citation systems. Keep in mind, though, that there are many more. In law, for example, a system called Bluebook is most prevalent. In chemistry, American Chemical Society style is often used. In journalism, the Associated Press Stylebook, which is revised annually, is revered. You can learn more about different styles at

Finally, though sites like Purdue OWL and other library pages are great general references, it is important to remember that they are not always accurate. The style guides change in subtle ways from edition to edition (especially as new types of online sources emerge), so students should always be encouraged to consult the actual style guides.