A style guide is a book that outlines the “rules” necessary to follow for any one kind of writing. These rules may be about simple things like grammar and punctuation, or more substantive questions about citation, layout, or format. Some guides may also speak to style as it applies to content and voice. Below is a primer on style guides. Note that these are U.S.-based style guides, and international users are sure to have their own rules to follow and books to buy!

Major Style Guides

• Associated Press. Associated Press is the go-to style for journalists and news writing. Sometimes this covers magazine writing, too, but each title is different. AP Style was originally written with the news wire in mind, and so symbols and “extras” like italics and underlining are kept to a minimum. For example, Latin names are printed without their accents in straight AP Style (although many publications correct this in their house style).
• Chicago Manual of Style. CMS is the standard for book publishing, both fiction and non-fiction. It is not generally used for scholarly publishing (journals and research), although it is sometimes used for history. CMS is currently in its 15th edition. For a more compact approach to CMS, take a look at the Turabian style, below.
• MLA. The Modern Language Association style is almost exclusively used in the academic world, and applies mostly to literature and humanities. This is likely the style first introduced to most writing students and undergrads. It does carry some similarities to CMS teachings.

• American Psychological Association. The APA carries its own standard for the social sciences such as psychology, sociology, education and policitcs. (Although the American Sociological Association produces a style guide specifically for sociology, see below). APA style is sometimes used for engineering and business work, too.
• Turabian. Turabian style is named after the book’s author, Kate Turabian, and focuses on style in research work. It is used for the research or academic arm of many subjects. In fact, many grad and undergrad students will be directed to use Turabian despite the availability of another system in their discipline. The CMS actually refers students to Turabian, and many will find it much easier to navigate, anyway.

Other Styles

• AMA. The American Medical Association is currently in its 10th edition of the AMA style guide, printed by Oxford Press. Of course, this is the go-to manual for health, medicine and biology subjects, unless…
• NLM. The National Library of Medicine has an online-only style guide that is often used in some of the AMA disciplines.
• CSE. The Council of Science Editors Manual covers natural sciences and biology.
• ACS. The American Chemical Society got in on the act with a style guide specifically for (you guessed it!) chemists. Chances are that if you write about chemistry, you already know about this guide, but here's a crib sheet on ACS style if you ever need it.
• ASA. Is the American Sociological Association trying to sway some former APA users? Maybe, although the APA still seems to be the more popular, even with more sociological-oriented disciplines.
• Bluebook.Bluebook citation is “it” for the legal profession, and, I'm told, yet another headache for law students. My gift to them: a Bluebook cribsheet.
• Harvard Style. Also called the Author-Date style of citation, but not a fully published guide per se.

There you go, a sampling of the most common style guides out there. Chances are, as a freelance writer, you won't need to go much further than the first 2-3 in the list, unless you have a niche specialization.