AP vs. Chicago: Which Style Guide Is Right for You?
First published in 2019
I originally wrote this article in 2019, when the Chicago Manual of Style was in its 17th edition. In September 2024, the University of Chicago Press released the 18th edition — described by its own publisher as the most extensive revision in a generation.
I'm updating this resource because it deserves to be current, and because it remains one of the most-read articles on this site. If you've been here before, there's new information worth sticking around for.
What is a style guide, and why does it matter for your book?
A style guide is exactly what it sounds like: a set of rules that govern how you write.
Capitalization, punctuation, numbers, titles, quotation marks — a style guide tells you how to handle all of it consistently, so your writing doesn't undermine its own authority by being inconsistent.
For nonfiction authors specifically, this matters more than most people realize. Your book goes through multiple hands (developmental editor, beta readers, copyeditor, proofreader, typesetter, etc.), and everyone needs to be working from the same rulebook. If you've never established which guide you're following — and which rules you’re breaking — you're essentially asking each person on your team to make it up as they go. That creates friction, inconsistency, and a finished product that feels slightly off in ways readers may not be able to name but will absolutely feel.
The two guides that dominate traditional publishing are the AP Stylebook and the Chicago Manual of Style. They're not interchangeable, and choosing the wrong one for your type of writing is one of the most common mistakes authors make before they even start working with an editor.
AP vs. Chicago
You’ve got options. And normally, that’s a good thing! But how are you supposed to know whether to use the AP Stylebook or the Chicago Manual of Style? Luckily, it’s pretty easy to determine which guide is right for you by asking yourself a few simple questions.
Am I writing a fiction or nonfiction novel or another type of long-form content?
Am I writing for newspapers, magazines, blogs, or creating short-form content?
If you answered yes to #1, you should use the Chicago Manual of Style. If you answered yes to #2, you should use the AP Stylebook.
The correct style guide for you depends on what each guide was built for. The AP Stylebook was created by the Associated Press, a global news organization, to help reporters produce clear, consistent copy on tight deadlines. It's compact, direct, and built for brevity. Many of the style guidelines result from how words are printed in columns in a physical newspaper. The Chicago Manual of Style was created by the University of Chicago Press and is built for long-form publishing — books, journals, academic writing. It's comprehensive, detailed, and built for permanence.
Both guides are updated regularly to reflect changes in our language, culture, and how we communicate. If you want to make sure you’re publishing content that reflects current best practices, go ahead and take a look at your style guide.
Who publishes the Chicago Manual of Style, and why does it get updated?
The Chicago Manual of Style is published by the University of Chicago Press, which has been putting out editions since 1906. The 18th edition, released in September 2024, is the most recent — and, by the Press's own description, the most extensive revision in a generation.
Style guides get updated because language does. The way we write about gender, technology, sourcing, inclusive language, and even the internet has shifted significantly in the last decade. A style guide that doesn't keep pace becomes a liability rather than a resource. This is especially true for nonfiction authors writing about contemporary subjects. If your editor is working from an outdated guide, your manuscript is being held to outdated standards.
The 17th edition was published in 2017. Seven years is a long time in publishing, and the 18th edition reflects it.
What changed in CMOS 18?
The changes in the 18th edition are genuinely significant, not just minor tweaks. Here's what nonfiction authors need to know:
Title case got a rule change. In the 17th edition, prepositions were only capitalized in titles if they were the first word. The 18th edition changed the threshold: any preposition of five or more letters is now capitalized. So "without," "about," and "throughout" get capitals in a title now. "What This Book Is About" becomes correct under CMOS 18 where it wasn't before.
"Headline style" is now called "title case." Minor terminology shift, but worth knowing if you've been using the old language with your editor or designer.
Publication location is no longer required in citations. Previously, a bibliography entry had to include the city of publication (New York: Penguin, 2019). Under CMOS 18, you can drop the location entirely (Penguin, 2024). Cleaner and more practical for modern publishing.
AI-generated content now has citation guidance. The 18th edition includes specific guidance on how to cite AI-generated text and images, and expanded discussion of AI and copyright. This didn't exist in the 17th edition, for obvious reasons.
Inclusive language got a full expansion. CMOS 18 includes updated and expanded guidance on pronoun use, singular they, and inclusive language more broadly — including new coverage of Indigenous languages (now capitalized). For nonfiction authors writing about people, communities, or contemporary issues, this section is essential reading.
Singular "they" is now more broadly endorsed. The 18th edition expanded the guidance to include use of singular and non-binary they in more contexts. If you've been writing around this question, CMOS 18 gives you clearer permission to use it.
Fiction writers got more support. New sections on dialogue, speaker tags, interrupted speech, and text message formatting were added — a reflection of how much fiction writing has become part of the broader CMOS conversation.
If you're working with an editor who references the Chicago Manual of Style, it's worth asking which edition they're using. The 17th is still widely used and perfectly valid — CMOS Online keeps it available alongside the 18th — but if you're publishing in 2025 or later, aligning to the 18th edition is the current standard.
5 key differences between AP and Chicago style
While the two guides agree on the fundamentals of grammar and mechanics, there are specific areas where they diverge. Getting these things wrong can look sloppy to an editor or reader who knows the difference; that is, of course, unless you’re breaking the rules on purpose.
1.Oxford Comma
This is the one that starts arguments at dinner parties, and for good reason. The Oxford comma (also called the serial comma) is the comma that comes before "and" or "or" in a list of three or more items.
Chicago uses it. AP doesn't — except when omitting it creates genuine ambiguity.
Chicago: I had dinner with my parents, Snoop Dogg, and Martha Stewart.
AP: I had dinner with my parents, Snoop Dogg and Martha Stewart.
For the record: I'm firmly, unapologetically team Oxford comma, and if you're writing a book under Chicago style, it's not optional.
And because of the clarity provided by the Oxford comma (no, Snoop Dog and Martha Stewart are not my parents), many brands will amend their in-house style to include the use of the Oxford comma even if they primarily use AP style.
It’s also worth noting (and I can’t believe I need to write this) that the use of the Oxford comma is not a “clear giveaway” that someone used AI to write for them.
2.Em Dash
Both guides use the em dash. The difference is spacing — and it comes directly from each guide's publishing origins. Remember when I said AP style was for newspapers and Chicago style was for long-form publication? In newspaper publication, there is limited real estate in a single line of text, which makes layout and readability extra important. Because of this, AP style adds spaces around the em dash while Chicago style does not.
AP: Martha made pot roast for dinner — my favorite.
Chicago: Martha made pot roast for dinner—my favorite.
If you're writing a book and you're using spaces around your em dashes, your copyeditor will fix every single one. If you want to save everyone some time, go ahead and Ctrl + f and replace all of those spaces. If you want to keep the spaces, you better have a good reason and you should let your editor know before they start working on your book.
3.How titles of works are styled
This one trips up authors and content creators who move between platforms.
Chicago italicizes titles of major works — books, films, albums, publications. AP uses quotation marks instead, largely because older print technology didn't support italics in news wire copy.
Chicago: She couldn't put down Educated.
AP: She couldn't put down "Educated."
For book authors, this is firmly Chicago territory: your title and the titles you reference throughout your manuscript are italicized.
4.Title capitalization
Under the old CMOS 17 rule, prepositions were lowercase in titles unless they were the first word. Under CMOS 18, the new rule is that prepositions of five or more letters get capitalized. AP capitalizes all words of four or more letters in a title.
AP: How To Get From Here to There
CMOS 18: How to Get from Here to There (short prepositions still lowercase)
CMOS 18 (with longer prepositions): Writing About Your Life, Writing Without Fear
5.Numbers
This is the rule that trips authors up most often, especially those who write about data, research, or anything quantitative.
AP spells out numbers one through nine, then uses numerals from 10 onward. Chicago spells out numbers zero through one hundred, then uses numerals from 101 onward.
AP: There were 22 people in the workshop.
Chicago: There were twenty-two people in the workshop.
If you're used to writing for digital publications or journalism and you switch to a book project, this is one of the adjustments that will touch nearly every page of your manuscript. This is also a rule that can be changed based on a publisher’s in-house style guide, depending on your industry, genre, or audience.
What are in-house style guides?
Here's a practical note that doesn't come up in most style guide comparisons: most publishers, publications, and brands maintain their own in-house style guide that sits on top of whichever base style they use.
An in-house style guide documents the exceptions to the rules in the places where a specific publication or publisher has decided to deviate from AP or Chicago, or where they've made a call on something neither guide addresses clearly. If you're working with a traditional publisher or a hybrid press, ask for their style guide early. If you're self-publishing, consider creating a simple one-page style sheet for your book that documents your decisions — your editor will thank you.
Use a style guide for consistent writing
Now that you know the difference between the AP Stylebook and the Chicago Manual of Style, you can go forth and write with confidence. Of course, there are exceptions to every rule, and the company you’re writing for may have its own style preferences.
You know what? That’s totally fine. Just make sure you communicate with your editor, get a copy of the in-house or preferred style guide, and never be afraid to ask questions. Whether you’re crafting the next great novel, drafting articles to pitch to online publishers, or developing content for your blog, using a style guide is the best way to ensure your writing is clear and correct.
If you need help determining which style guide is right for you or you want to develop a style guide for your own brand, send me an email; I’m happy to guide you in the right direction.
