AP vs Chicago Title Case: Tested Differences

The quickest difference is a four-letter preposition

Run the same input through both profiles: “a guide to writing with and without fear.” Tool Packer’s AP profile returns “A Guide to Writing With and Without Fear,” while its Chicago profile returns “A Guide to Writing with and Without Fear.” The only visible difference is With versus with, making the result useful for checking the implemented threshold without changing the surrounding grammar.

In the AP profile, recognized prepositions of three letters or fewer are lowercased internally, so to stays lowercase but with is capitalized. Chicago lowercases recognized prepositions through four letters, so both to and with are lowercase. Both capitalize Without because it is longer than either profile’s threshold.

Conjunction and exception lists also diverge

Tool Packer’s AP-oriented list lowercases the coordinating conjunctions for, and, nor, but, or, yet, and so, plus as and if when they occur internally. Its Chicago list lowercases for, and, nor, but, and or, along with as, but explicitly capitalizes If, Yet, and So. A title containing those words can therefore differ even when it has no four-letter preposition.

These decisions come from explicit word sets, not a full grammatical parser. A spelling can act as a conjunction, preposition, adverb, particle, or another part of speech depending on context. The explanation option reports which project rule fired, but an editor must decide whether that mechanical classification fits the sentence.

• Both profiles lowercase internal and and but.
• AP lowercases internal if, yet, and so; Chicago capitalizes them.
• Both lowercase internal as in the implemented profiles.

Where the profiles agree

Both implementations capitalize the first word and the first word after a colon, em dash, or line break. Both also capitalize the final word, even if the same word would ordinarily be lowercase in the middle. Internal articles and recognized prepositions through three letters are lowercased by both profiles, so many short titles produce identical output.

The profiles share Tool Packer’s acronym, proper-term, custom-term, all-caps preservation, and compound handling. That common processing restores canonical spellings such as GitHub and API and applies the project’s predictable rule to hyphenated compounds. These shared conveniences are useful, but they are not evidence that every name or compound follows every provision in either full style guide.

• Keep the original title available for comparison.
• Use explanations to identify the exact mechanical rule.
• Add authoritative brand spellings as request-scoped custom terms.

Choose by governing publication, not preference

If a publisher, employer, client, course, or house style names AP or Chicago, that requirement determines the profile. Check important titles against the current source because official guidance, editions, and house exceptions can change while Tool Packer updates only when its open-source rules are deliberately revised. Formatting choices such as italics, quotation marks, and headline punctuation remain separate from capitalization.

Tool Packer is independent and is not affiliated with, endorsed by, or certified by the Associated Press or the University of Chicago. This page describes a transparent converter implementation and a reviewed comparison boundary. It does not reproduce either proprietary manual or promise that an automated result resolves syntax, specialized terminology, or every editorial exception.

• Record the selected profile in repeatable workflows.
• Verify edition-sensitive or high-stakes copy manually.
• Let documented house rules override the converter.