Title Case and Sentence case both get used constantly, often inconsistently within the same document, because most people have never learned the actual rule distinguishing them.
Sentence case: the simpler rule
Only the first word (and any proper nouns) get capitalized — the rest of the sentence follows normal capitalization rules exactly as you'd write regular prose. This is the standard for body text, most modern web UI labels, and increasingly for headlines at many publications that have moved away from traditional title case.
Title case: more nuanced than "capitalize every word"
Traditional title case capitalizes major words but lowercases minor ones — articles (a, an, the), coordinating conjunctions (and, but, or), and short prepositions (in, on, of, to) — unless they're the first or last word of the title, in which case they're capitalized regardless of their normal minor-word status. This is where most manual title-casing goes wrong: capitalizing every single word regardless of its grammatical role.
Why style guides disagree on the details
Different style guides (AP, Chicago, APA, MLA) draw the "minor word" line in slightly different places — some treat prepositions of four or more letters as major words worth capitalizing, for instance, where others don't — so "correct" title case can vary by which style guide you're actually following.
Where each is the accepted convention
Title case remains standard for book and movie titles, and many print headlines. Sentence case has become the dominant convention for web headlines, UI button and menu text, and most professional email subject lines in modern style.
Convert reliably
A case converter applies these capitalization rules consistently, including the minor-word exceptions that trip up manual title-casing most often.
The bottom line
The main manual mistake is capitalizing every word in title case — genuine title case selectively lowercases short function words, and that's exactly the part a tool gets consistently right where memory doesn't.
Try it yourself
Put this into practice with our case converter.