Chicago title case capitalizes the first and last word, every major word, and – since the 18th edition – every preposition of five letters or more. Prepositions of four letters or fewer stay lowercase, as do articles and coordinating conjunctions.
That five-letter rule is new. If you learned Chicago from the 17th edition or earlier, the rule you learned was to lowercase prepositions at any length.
What the 18th Edition Changed
For decades, Chicago's distinguishing feature was that it lowercased prepositions regardless of length. That is what produced the classic Chicago rendering of long prepositions in book titles, and what separated it most sharply from AP.
| Title | Chicago 17 and earlier | Chicago 18 |
|---|---|---|
| much ado about nothing | Much Ado about Nothing | Much Ado About Nothing |
| the bridge over the river | The Bridge over the River | The Bridge over the River |
| a journey through the alps | A Journey through the Alps | A Journey Through the Alps |
| notes toward a theory | Notes toward a Theory | Notes Toward a Theory |
The second row is worth reading twice: "over" has four letters, so it stays lowercase under both editions. The change only affects prepositions at five letters and above.
Where the Line Falls
Reading the Chicago column: everything at four letters or fewer is lowercase, everything at five or more is capitalized.
| Word | AP | APA | Chicago | MLA |
|---|---|---|---|---|
| of (2 letters) | Lowercase | Lowercase | Lowercase | Lowercase |
| for (3 letters) | Lowercase | Lowercase | Lowercase | Lowercase |
| with (4 letters) | Capitalize | Capitalize | Lowercase | Lowercase |
| over (4 letters) | Capitalize | Capitalize | Lowercase | Lowercase |
| about (5 letters) | Capitalize | Capitalize | Capitalize | Lowercase |
| under (5 letters) | Capitalize | Capitalize | Capitalize | Lowercase |
| through (7 letters) | Capitalize | Capitalize | Capitalize | Lowercase |
Chicago now sits one letter above AP and APA, and well above MLA, which lowercases prepositions at any length. MLA vs APA covers the two academic guides against each other.
The Rules That Did Not Change
- Capitalize the first and last word of the title and of any subtitle, whatever they are.
- Capitalize nouns, pronouns, verbs, adjectives and adverbs at any length – including Is, Be, It and Do.
- Lowercase the articles a, an, the in middle positions.
- Lowercase the coordinating conjunctions and, but, for, or, nor, so, yet.
- Capitalize subordinating conjunctions – Although, Because, If, When, That – as major words.
- Lowercase to in an infinitive: How to Cook Everything.
- Capitalize the first word after a colon.
The full rule set with side-by-side comparisons lives in the capitalization rules guide. For the general question of which words stay lowercase in any style, see what words are not capitalized in a title.
When to Use Chicago
Chicago is the default for book publishing and for a large share of formal editorial work in the United States. Trade publishers, university presses, and many magazines specify it in their house style sheets.
| Context | Usual guide |
|---|---|
| Books, university presses, history writing | Chicago |
| News copy, press releases, marketing | AP |
| Psychology, education, social sciences | APA |
| Literature, languages, humanities coursework | MLA |
If a publisher names Chicago but your source material was styled under an older edition, the preposition threshold is the thing to re-check. Paste a list of titles into the batch headline checker with Chicago selected and it flags every line that disagrees.
Frequently Asked Questions
Select Chicago in the free Title Case Converter and it applies the 18th edition's 5-letter threshold, marking every word it changed and why.
Open the converter with a prefilled example and adapt it to your headline.
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About Sarah Klein
Focuses on APA and academic formatting, helping researchers and students navigate title case rules with confidence.
