APA capitalizes “Upon” internally because it has four letters. Chicago 18 and MLA normally lowercase it internally as a preposition. This guide keeps style-specific rules separate and marks grammatical ambiguity where the same spelling can play different roles.
ReviewedEvidence4 sourcesSectionWriting & Style
Quick answer
APA capitalizes “Upon” internally because it has four letters. Chicago 18 and MLA normally lowercase it internally as a preposition.
Key details
APA 7Capitalize internally under the four-or-more-letter rule
Automation NoteBoundary rules override ordinary internal treatment. Tokens with multiple grammatical roles remain review-only unless context is established safely.
Chicago 18Lowercase internally as a preposition of four letters or fewer
MLA 9Normally lowercase internally as a preposition
TermUpon
Word Classpreposition
Style-by-style guidance
APA 7
Capitalize internally under the four-or-more-letter rule.
Chicago 18
Lowercase internally as a preposition of four letters or fewer.
MLA 9
Normally lowercase internally as a preposition.
Important caveats
Automation boundary
Context can help identify the word’s role, but an ambiguous token should not be forced into a single title-case result.
Further guidance
Quick rule
APA capitalizes “Upon” internally because it has four letters. Chicago 18 and MLA normally lowercase it internally as a preposition.
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.
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.
APA title case capitalizes the first word, the first word of a subtitle, major words, and words of four letters or more while keeping short minor words lowercase in ordinary mid-title positions.
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.
MLA 9 describes title style as capitalizing the first word, last word, and all principal words. MLA’s public Style Center also documents the traditional lowercase treatment of internal prepositions, but its detailed preposition article is edition-labeled for MLA 8, so this engine does not yet treat the full MLA 9 converter profile as complete.
APA capitalizes “Against” internally because it has at least four letters. Chicago 18 also capitalizes it because prepositions of five letters or more are capitalized. MLA ordinarily lowercases internal prepositions, while any principal-word use needs grammatical classification before an automated decision.
APA capitalizes “Among” internally because it has at least four letters. Chicago 18 also capitalizes it because prepositions of five letters or more are capitalized. MLA ordinarily lowercases internal prepositions, while any principal-word use needs grammatical classification before an automated decision.
Usually no in the middle of a title. “at” is a short preposition: APA lowercases short prepositions of three letters or fewer, Chicago 18 lowercases prepositions of four letters or fewer, and MLA title style normally treats internal prepositions as minor words. The answer depends on the style guide and position, so this page separates APA 7, Chicago 18, and MLA guidance instead of applying one generic title-case list.
Usually no in the middle of a title. “by” is a short preposition: APA lowercases short prepositions of three letters or fewer, Chicago 18 lowercases prepositions of four letters or fewer, and MLA title style normally treats internal prepositions as minor words. The answer depends on the style guide and position, so this page separates APA 7, Chicago 18, and MLA guidance instead of applying one generic title-case list.
