Chicago Title Case (18th Edition)
Chicago 18 uses title case for titles of works and changed a long-standing rule: prepositions of five letters or more are now capitalized. Shorter prepositions generally remain lowercase internally, while first and last words are capitalized and “to” remains lowercase as an infinitive marker in ordinary mid-title use.
Chicago 18 capitalizes first and last words and principal words. A major 18th-edition change is that prepositions of five letters or more now take an initial capital; shorter internal prepositions remain lowercase. “To” stays lowercase as an infinitive marker in ordinary mid-title position.
Key details
Important caveats
Automation boundary
Chicago title case still depends on grammatical analysis in some cases. For example, a word such as “than” can be treated differently depending on whether it functions as a preposition or a conjunction. This guide therefore keeps the Chicago converter profile disabled until those ambiguity tests are explicit.
Further guidance
Edition Change
The 18th edition changed Chicago’s terminology from “headline style” to “title case” and changed the preposition threshold: prepositions of five letters or more are now capitalized.
Practical Application
Apply Chicago 18 title case by capitalizing the first and last words and principal words, while applying the edition’s specific rules for prepositions, conjunctions, infinitival “to,” compounds, and grammatical ambiguity. In Chicago 18, internal prepositions of five letters or more are capitalized; shorter internal prepositions generally remain lowercase.
Reader Check
Have you identified the grammatical role of short words and compounds rather than capitalizing solely by word length?
Short Prepositions
Internal prepositions of four letters or fewer generally remain lowercase under Chicago 18 unless another rule applies, such as first- or last-word position.
To In Infinitives
Chicago keeps “to” lowercase when it marks an infinitive in ordinary mid-title position; it is capitalized when it is the first or last word.
Worked Example
“A Walk Through the Woods” capitalizes “Through” under Chicago 18 because it is a preposition of five or more letters, while an internal infinitival “to” remains lowercase in ordinary use.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Headlines and Titles of Works — Chicago Q&A (opens in a new tab)The Chicago Manual of Style / University of Chicago Press · Chicago title-case rules and current-edition updates
- What's New in the 18th Edition (opens in a new tab)The Chicago Manual of Style / University of Chicago Press · Chicago 18 title-case terminology and five-letter-preposition change
- Headlines and Titles of Works — “to” in infinitives (opens in a new tab)The Chicago Manual of Style / University of Chicago Press · Chicago treatment of “to” in titles, including infinitives and edge positions