Most academic disciplines expect writers to credit sources using a standardized citation style, and three of the most widely used systems are APA, MLA, and Chicago. The choice of style usually depends on the field. APA 7th edition is primarily used in the social sciences, including psychology, education, and business; MLA 9th edition is primarily used in the humanities, including literature, philosophy, and cultural studies; and Chicago/Turabian style is primarily used in history, some humanities, and certain social science disciplines, where it offers two systems, a Notes-Bibliography system that uses footnotes or endnotes, and an Author-Date system that resembles APA in its in-text citations.
Each style has distinctive conventions for in-text references. In APA 7th edition, the author's last name and year of publication appear together in parentheses, as in (Smith, 2022), and a page number is added for direct quotations, as in (Smith, 2022, p. 45). MLA 9th edition uses the author's last name and page number with no comma, written as (Smith 45). Beyond the in-text citation, every style requires a list of full references at the end of the paper. APA titles this page References, lists entries alphabetically by author's last name, applies a hanging indent, and requires double spacing. MLA titles the corresponding page Works Cited, centered at the top. The core elements of an MLA 9th edition Works Cited entry are the author, the title of the source, the title of the container, contributors, version, number, publisher, publication date, and location. Chicago uses either a Bibliography (in Notes-Bibliography style) or a Reference List (in Author-Date style), depending on which system the writer adopts.
Citation styles also share several practical features. A hanging indent, in which the second and subsequent lines of an entry are indented by 0.5 inches, is used in APA References, MLA Works Cited, and Chicago Bibliography pages. In APA 7th edition, a DOI, or Digital Object Identifier, a unique alphanumeric string assigned to a digital document, provides a permanent link to the source and is required in reference entries. The abbreviation "et al." (Latin for "and others") is used in APA 7th edition in-text citations for works with three or more authors, following the first author's name. Knowing these shared and distinct features helps writers transition between styles when they move into a new field or submit to a different venue.