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.
ReviewedEvidence3 sourcesSectionWriting & Style
Quick answer
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
Converter Statusnot yet enabled; grammatical ambiguity and compound tests incomplete
EditionChicago Manual of Style, 18th edition (2024)
Preposition ThresholdCapitalize prepositions of five letters or more; lowercase shorter internal prepositions
Profile Idchicago-18
To In Infinitivelowercase internally
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.
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.
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.
Hyphenated compounds cannot be capitalized safely with one universal “capitalize everything after a hyphen” rule. APA explicitly capitalizes the second part of hyphenated major words, while Chicago 18 changed its prefix-compound treatment and still applies grammatical title-case rules to individual elements.
Short words such as “up,” “out,” and “on” can change capitalization when their grammatical role changes. Chicago’s own examples capitalize “Up” or “Out” when those words function adverbially in a phrasal verb, while ordinary prepositional uses can remain lowercase.
Prepositions are one of the clearest places where title styles diverge. APA uses a four-letter major-word threshold, Chicago 18 capitalizes prepositions of five letters or more, and MLA editorial guidance generally lowercases internal prepositions regardless of length.
The first word of a subtitle is treated as a new capitalization boundary in major title-case systems. APA explicitly capitalizes the first word of a subtitle, Chicago capitalizes the word after a title/subtitle colon, and MLA has published the same treatment in its Style Center guidance.
The word “and” is normally lowercase in the middle of a title because it is a coordinating conjunction, but first-word, last-word, and style-specific rules can change the answer.
“Of” is normally lowercase in the middle of a title in APA 7, Chicago 18, and MLA-style title capitalization because it is a short preposition. First- or last-word position can override the ordinary middle-of-title treatment.
“So” is normally lowercase in the middle of APA and Chicago title case when it functions as one of the short/common coordinating conjunctions covered by those styles. It is capitalized when a positional rule applies, such as appearing first in the title.
“To” is normally lowercase in the middle of titles under APA, Chicago, and MLA-style capitalization when it functions as a short preposition; Chicago also explicitly keeps “to” lowercase as an infinitive marker in ordinary mid-title position. Edge position can change the result.
