Sentence case vs title case

Which words to capitalise in a heading, why the three style guides disagree, and the rule that stops you having to care.

The two you actually choose between

Sentence case capitalises the first word and nothing else, except proper nouns. How to fix a JSON syntax error.

Title case capitalises the important words and leaves the small ones alone. How to Fix a JSON Syntax Error.

Both are correct. The question is which one your publication uses, and then using it consistently, inconsistency is far more noticeable than either choice.

Which to pick

Sentence case has quietly won on the web. Google, Apple, GOV.UK, Wikipedia and most modern documentation all use it for headings. It reads faster, it handles long headings better, and it removes an entire category of decision from whoever is writing.

Title case still holds in American journalism, book titles, academic citations, and a lot of marketing copy. It signals formality, which is sometimes exactly what you want.

If nobody has decided for you: sentence case. It is the lower-effort choice and the one that ages better.

Why title case is harder than it looks

"Capitalise the important words" is not a rule a computer can follow, so every style guide has written down its own approximation, and they disagree.

	AP	Chicago	APA
Words of 4+ letters	Capitalise	Capitalise	Capitalise
Short prepositions (of, in, to)	lower case	lower case	Capitalise if 4+ letters
Is, Are, Be	Capitalise	Capitalise	Capitalise
First and last word	Always	Always	Always
After a colon	Capitalise	Capitalise	Capitalise
Length threshold	4 letters	not length-based	4 letters

So A Guide to the Galaxy in AP is A Guide to the Galaxy in Chicago, but Words With Friends is Words with Friends in AP and Words with Friends in Chicago too, while APA gives you Words With Friends.

This is why two people hand-capitalising the same heading produce different results and both think the other is wrong.

The words that stay lowercase

Under AP and Chicago, these stay down unless they are first or last:

Articles: a, an, the
Coordinating conjunctions: and, but, or, nor, for, yet, so
Short prepositions: at, by, in, of, on, to, up, via

Two traps in that list. For and so are conjunctions here, not prepositions they stay down. And up, off and out go up when they are part of a verb: Turning Off the Lights, not Turning off the Lights. No automatic converter gets that right, because it needs to know what the sentence means.

Acronyms are the thing converters break

This is where an automatic conversion does real damage. Run Fix a JSON error through a naive title-caser and you get Fix a Json Error. Sentence case is worse: fix a json error.

Any converter worth using has to leave a run of capitals alone, JSON, PDF, API, HTTP, NASA, iOS, and the good ones let you add your own. If yours does not, it is only safe on text with no acronyms in it, which is not most headings.

The other cases, and what they are for

Outside prose, case is a naming convention and the choice is made for you by the language:

Name	Looks like	Where
camelCase	`userName`	JavaScript, Java variables
PascalCase	`UserName`	Classes, React components, C#
snake_case	`user_name`	Python, Ruby, SQL columns
SCREAMING_SNAKE	`USER_NAME`	Constants, environment variables
kebab-case	`user-name`	CSS classes, URLs, filenames
dot.case	`user.name`	Config keys, namespaces

The one with a real-world consequence is kebab-case for URLs. Search engines treat hyphens as word separators and underscores as joiners, so user-name reads as two words and user_name reads as one. Use hyphens in anything that becomes a web address.