“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.
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Quick answer
Usually lowercase “so” in the middle when it is functioning as a coordinating conjunction under APA or Chicago title case. Capitalize it at the beginning of a title or when another rule independently requires capitalization. Do not infer the result from spelling alone if “so” has another grammatical function.
Key details
APA 7lowercase as short conjunction internally
Chicagolowercase as common coordinating conjunction internally
Wordso
Word Classcontext-sensitive; commonly coordinating conjunction
Style-by-style guidance
APA 7
APA lists “so” among short conjunctions that are lowercase in title case unless a boundary rule applies.
Chicago
Chicago identifies “so” as one of the seven common coordinating conjunctions that are normally lowercase internally.
Further guidance
Grammar Matters
“So” can also serve grammatical roles other than a coordinating conjunction. A converter should classify the context before applying a conjunction rule.
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.
“Yet” is normally lowercase in the middle of APA and Chicago title case when it functions as a coordinating conjunction. As with other minor words, title position and grammatical role can override a simple lowercase rule.
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.
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.
Coordinating conjunctions are a major source of title-case differences: APA lowercases short conjunctions, while current Chicago names a narrower lowercase set and capitalizes “so” and “yet.”
APA capitalizes “About” internally because it has at least four letters. Chicago 18 also capitalizes it when used as a preposition because current Chicago capitalizes prepositions of five letters or more. MLA treatment still depends on grammatical function, so prepositional and principal-word uses should not be collapsed into one rule.
APA capitalizes “Across” internally because it has at least four letters. Chicago 18 also capitalizes it when used as a preposition because current Chicago capitalizes prepositions of five letters or more. MLA treatment still depends on grammatical function, so prepositional and principal-word uses should not be collapsed into one rule.
APA capitalizes “After” internally because it has at least four letters. Chicago 18 also capitalizes it when used as a preposition because current Chicago capitalizes prepositions of five letters or more. MLA treatment still depends on grammatical function, so prepositional and principal-word uses should not be collapsed into one rule.
