The Chicago Manual of Style is the dominant style guide for American book publishing, academic writing, and literary journals. Its title case rules capitalize all major words while lowercasing short prepositions, articles, and coordinating conjunctions.
Chicago Style title case capitalizes nouns, verbs, adjectives, adverbs, and long prepositions in titles. Short prepositions such as "in," "on," "at," "by," and "of," articles, and coordinating conjunctions are lowercased unless they open or close the title.
The Chicago Manual of Style defines a short preposition as one of four letters or fewer. This threshold determines which prepositions appear in lowercase in properly formatted Chicago Style titles.
Chicago Style is the standard for books, academic journals, literary magazines, and university press publications. If you are submitting a manuscript to a publisher or writing for an academic journal, Chicago Style is almost certainly what your editor expects.
Scholars in the humanities, including historians, literary critics, and philosophers, default to Chicago Style because it is the standard taught in most graduate programs. Its detailed rules cover everything from footnotes and bibliography formatting to title capitalization.
The rule for prepositions in Chicago Style is based on word length. Prepositions of five or more letters, such as "about," "across," "between," and "through," are capitalized, while shorter ones like "in," "on," and "at" remain lowercase.
The infinitive "to" is always lowercase in Chicago Style, as are articles and coordinating conjunctions. The first and last words of a title are always capitalized, overriding all other rules about short words.
The most important difference between Chicago Style and AP Style is how they treat prepositions. Chicago only lowercases short prepositions of four letters or fewer, while AP lowercases all prepositions regardless of length.
Book publishers almost universally follow Chicago Style, while newspapers and wire services follow AP Style. The context of your writing determines which guide applies.