AP vs Chicago Style: The Headline Rules That Disagree

If you have ever worked on a publication that switched style guides, or onboarded with an editor who comes from a different one, you know that AP and Chicago aren't really two different rulesets — they're two different temperaments. AP wants brevity, predictability, and rules a sub-editor can apply under deadline pressure. Chicago wants typographic refinement and consistency across a 700-page book.

The places they disagree are concentrated, specific, and — to the people who care — vehemently held. This is a working reference to the disagreements that come up most often in headline writing, copy editing, and digital publishing.

Who uses which

Knowing your context settles half the arguments before they start:

• AP Style is the default in US newsrooms (AP itself, Reuters, most major US dailies), most US-based digital publications, corporate communications, and PR. Updated annually; the current edition is the AP Stylebook 56th edition (2022) with online updates.
• Chicago Manual of Style is the default in US book publishing, academic presses (most humanities and social sciences), and the higher end of editorial / longform digital publications (The Atlantic, Harper's, New York Times Magazine). Currently in its 17th edition.

If your house style sheet says "follow Chicago except where noted", that means: in the gaps, defer to Chicago. AP doesn't enter the picture.

If you're writing for a publication that hasn't told you which to use, the most reliable tell is to look at how they format dates and addresses in their existing content. AP writes "Sept. 12" and abbreviates months. Chicago writes "September 12" and spells them out.

The five biggest disagreements

1. Prepositions in headlines

This is the disagreement that produces the most actual edits.

AP: Capitalise prepositions of four or more letters. Lowercase three-letter and shorter prepositions.

AP: "How to Write with Confidence" — with capitalised (4 letters).

Chicago: Lowercase all prepositions regardless of length.

Chicago: "How to Write with Confidence" — with lowercased.

The four-letter cutoff in AP creates a small army of "questionable" prepositions: from, into, onto, upon, over, with, about, after, among, between, through. Chicago has none of this — all of them lowercase.

The reason for the difference: AP's rule was designed for editors making fast decisions on headlines. Chicago's rule was designed for typesetters who'd rather over-lowercase than under-lowercase.

2. Hyphenation in compound modifiers

This one is contested in both directions.

AP: Use a hyphen for compound modifiers before a noun, with some exceptions. Hyphenate after a noun only "to avoid ambiguity".

AP: "a well-known author" (before the noun) — "the author is well known" (after the noun).

Chicago: Similar rule, but Chicago is more permissive about not hyphenating familiar compounds. Chicago also has a famous 12-page table (the "hyphenation guide") that enumerates specific compounds and their default form.

Chicago's "third-grader" is hyphenated; AP's might be too, but AP is more likely to defer to ear. Chicago's "high school student" is unhyphenated (because "high school" is widely understood as a unit); AP's would be the same. Chicago's "well-known" follows the same rule as AP.

The genuine difference: when in doubt, AP hyphenates, Chicago doesn't. AP optimises for clarity to the reader; Chicago for clean typography.

3. After-colon capitalisation

AP: Capitalise the first word after a colon only if what follows is a complete sentence.

AP: "She had one rule: Always read the email twice." AP: "There are three flavours: vanilla, chocolate, and strawberry."

Chicago: Capitalise the first word after a colon only if what follows is two or more sentences or a formal question.

Chicago: "She had one rule: always read the email twice." (one sentence, lowercase) Chicago: "Here's the plan: We meet at noon. We leave at one." (two sentences, capitalise)

This is the disagreement that most often produces inconsistent published text, because writers usually default to AP-style "capitalise after colon if it's a sentence" without realising that Chicago wants more.

4. The Oxford comma

The most famous disagreement, partially resolved.

AP: Do not use the Oxford (serial) comma in simple lists.

AP: "I had eggs, toast and coffee."

Exception in AP: Use the Oxford comma when omitting it creates ambiguity.

AP: "I dedicate this book to my parents, Ayn Rand and God." (use the comma here.)

Chicago: Always use the Oxford comma.

Chicago: "I had eggs, toast, and coffee."

This is the disagreement most readers don't notice, and the one most editors will die on a hill defending. The argument for AP's no-Oxford default is space (matters in print headlines, less in digital). The argument for Chicago's always-Oxford is consistency.

A useful note: if you're writing in British English, neither rule technically applies — UK style is closer to AP (no Oxford comma by default) but with significant publication-by-publication variation. The Times doesn't use it. The Economist generally doesn't. The Guardian uses it inconsistently.

5. Numbers in body text

AP: Spell out numbers one through nine; use numerals for 10 and above. Always use numerals for ages, addresses, percentages, dollar amounts, and most temperatures.

AP: "She bought three apples. He bought 12 oranges. She is 7 years old."

Chicago: Spell out numbers one through one hundred and round numbers (e.g., two hundred, fifteen thousand). Use numerals for non-round numbers above 100.

Chicago: "She bought three apples. He bought twelve oranges. The audience held two hundred people. The article was 1,237 words."

AP's rule produces text full of numerals. Chicago's produces text full of spelled-out numbers. The difference is most striking in business writing, where Chicago's "twenty-five percent" looks deliberately archaic to an AP reader.

The smaller disagreements (that still matter)

Quotation marks

Both prefer double quotes for direct speech and single quotes for nested quotes. They agree on US conventions for placing commas and periods inside quotation marks ("inside", not "outside"). But:

• AP uses italic for titles of books, films, magazines.
• Chicago uses italic for books, films, magazines, ships, paintings, plays, and journals; quotes for shorter works like articles, songs, episodes.

This is mainly a problem when an AP-trained editor takes over a Chicago publication — they tend to over-quote and under-italicise.

Dashes

• AP: Em-dashes — no spaces around them.
• Chicago: Em-dashes — no spaces. (Agrees with AP.)

But AP tends to favour em-dashes for parenthetical asides, while Chicago is more comfortable with en-dashes for ranges. Pages 23–47 is Chicago-style. AP would write it as pages 23 to 47 in body text and 23-47 in tables. Em-dash and en-dash have different keyboard shortcuts, and getting the wrong one is one of the most common silent typos in published writing.

Abbreviations

• AP: Abbreviate month names with specific dates ("Sept. 12") but spell out when standalone ("the September issue"). Abbreviate states in datelines ("CALIF.") but spell out in body text ("California").
• Chicago: Spell out months always. Use the two-letter postal abbreviations for states (CA, NY) only with ZIP codes.

This is mostly invisible to readers but causes huge cleanup work when porting between styles.

Recent AP changes worth knowing

AP updates its stylebook annually. Recent changes (2022–2024) that catch editors out:

• "Email" is one word (was "e-mail" until 2011).
• "Website" is one word (was "web site" until 2010).
• "Internet" is lowercase (was capitalised until 2016).
• Singular "they" is officially accepted for unknown or non-binary singular subjects (changed 2017, expanded 2019).
• "%" is now preferred over "percent" in body text (changed 2019).
• Hyphens removed from many "co-" and "non-" compounds in 2019.

Chicago tracks the same trends but updates less frequently. The 17th edition (2017) was the most recent major update; smaller revisions appear in the online subscription.

A working rule for choosing between them

If your publication doesn't have a house style and you have to pick:

• For news writing, web content, marketing, and PR: AP. It's faster to apply, more familiar to most readers, and the established default for digital publishing.
• For long-form journalism, books, academic writing, and editorial prose: Chicago. The rules produce more elegant typography over book-length text.
• For technical documentation: Neither, really — the Microsoft Writing Style Guide, the Google Developer Documentation Style Guide, or O'Reilly's house style are all better fits.

A common, defensible compromise for blogs is AP for body text, Chicago for headings. This gets you AP's faster body-text rules (less spelling out of numbers, no Oxford comma) and Chicago's more deferential title-casing (lowercase prepositions across the board).

Using a converter

For batch conversion of headings between cases, the case converter handles the mechanics in one click. The Title Case option uses AP-style rules by default — it capitalises prepositions of 4+ letters. If you're writing in Chicago, you'll want to manually lowercase any 4+ letter preposition that AP would have capitalised. The other ten case styles (sentence case, camelCase, snake_case, etc.) are useful for the non-headline parts of editorial workflows.

The honest summary

AP and Chicago agree on the vast majority of rules. The disagreements are concentrated in: prepositions, the Oxford comma, after-colon capitalisation, hyphenation defaults, and the treatment of numbers in body text. Knowing which you're following matters less than picking one and applying it consistently.

The cardinal sin is mixed styling within a single piece — capitalising prepositions of 4+ letters in one heading and not the next, or Oxford-comma-ing some lists and not others. Either style is a defensible choice. Inconsistency isn't.