Result
It depends on the guide and on the job "so" is doing. As a conjunction joining two clauses ("The Store Closed, so We Went Home"), AP, APA, MLA, AMA, and Bluebook keep it lowercase, while Chicago capitalizes it. As an adverb meaning "very" or "to that degree" ("So Long," "Not So Fast"), it is a major word and every guide capitalizes it.
"So" (as a Conjunction) by Style Guide
| Style guide | "So" mid-title | Why |
|---|---|---|
| AP | Lowercase | Conjunctions of three or fewer letters stay lowercase |
| APA | Lowercase | APA lists "so" among the short conjunctions it lowercases |
| Chicago | Capitalize | Chicago lowercases only and, but, for, or, and nor, so "so" is capitalized |
| MLA | Lowercase | Coordinating conjunctions stay lowercase |
| AMA | Lowercase | Coordinating conjunctions stay lowercase |
| Bluebook | Lowercase | Conjunctions of four or fewer letters stay lowercase |
The Chicago difference is deliberate. Its rules lowercase only five coordinating conjunctions (and, but, for, or, nor), which leaves "so" and "yet" capitalized. MLA's list includes all seven.
The Same Headline in Four Styles
| Style | "the store closed so we went home" |
|---|---|
| AP | The Store Closed so We Went Home |
| APA | The Store Closed so We Went Home |
| Chicago | The Store Closed So We Went Home |
| MLA | The Store Closed so We Went Home |
The converter treats "so" as a conjunction. If your "so" means "very," capitalize it yourself in any style.
"So" as an Adverb
Most titles that use "so" use it as an adverb, and adverbs are capitalized everywhere: "So Long, and Thanks for All the Fish," "Why Is This So Hard," "Not So Fast." Ask whether "so" could be replaced with "very" or "that." If it could, it is an adverb.
Related Words
"Yet" works the same way: lowercase as a conjunction outside Chicago, capitalized as an adverb ("Not Yet"). For the other conjunctions, see "and" and "but".
Real Titles With "So"
- So Long, and Thanks for All the Fish (Douglas Adams): first word, adverb
"So" in Sentence Case
APA reference lists, many news sites, and most UI text use sentence case instead of title case. There, only the first word and proper nouns get capitals, so "so" is lowercase unless it starts the title: "The store closed so we went home." Pick one of the two styles for a whole set of headings rather than mixing them.
Quick Answers
Should "so" be capitalized in a title?
As an adverb ("So Long"), yes, everywhere. As a conjunction, only in Chicago style.
Is "so" capitalized in Chicago style?
Yes. Chicago lowercases only and, but, for, or, and nor, so "So" is capitalized.
Is "so" capitalized in APA?
Not as a conjunction. APA lowercases short conjunctions, and its list includes "so."