But is the little joining word meaning "however" or "except": "I'd come, but I'm working late." Butt, with two t's, is a noun meaning the backside, the thick end of an object, or the target of a joke, and a verb meaning to strike with the head: "The goat butted the gate." If the word is holding two ideas together, it takes one t.
The core difference
These two sound identical in most accents, which is why the mistake survives spellcheckers only when the wrong one happens to be a real word — and here both are. But is grammatical machinery: a conjunction, and occasionally a preposition. Butt is a content word: a thing, or an act of shoving with the head.
Using "but"
Most often it links two clauses that pull against each other:
- The flat is tiny, but the light is wonderful.
- She apologised, but she did not explain.
It also works as a preposition meaning "except": Everyone but Priya had read the report. And in speech it can soften a sentence opener — But that's the whole point. Starting a sentence with but is fine; it has been ordinary in published English for centuries.
Using "butt"
The noun covers several senses that all involve a blunt or thick end, plus one about targets:
- Body: He landed on his butt. (chiefly American; British English usually says bum or backside)
- The thick end of a thing: the butt of a rifle, a cigarette butt
- A target: He was the butt of every joke in the office.
- A water butt — a barrel for collecting rainwater.
As a verb it means to ram or push with the head, and it gives us the phrasal verb butt in, meaning to interrupt: Sorry to butt in, but the taxi's here.
The test
Try replacing the word with however or except. If either fits, you want but with one t. If the word names a thing you could point at, or describes a shove, you want butt. A second memory hook: butt is the fatter word for the fatter end.
The mistake people actually make
The commonest slip goes the other way round: writing but in for butt in, and but of the joke for butt of the joke. Both of those fixed phrases take the double t, because they belong to the target-and-ramming sense, not the however sense. Watch too for butt heads (to clash with someone) and butt out (to stop interfering) — again, two t's.
Frequently Asked Questions
Is it "butt in" or "but in"?
Butt in, with two t's. The phrasal verb comes from the sense of shoving with the head, so it keeps the double t: "Don't butt in while she's speaking." The only time you'd write "but in" is when "but" happens to sit next to a separate "in", as in "nowhere but in London".
Is it "the butt of the joke" or "the but of the joke"?
The butt of the joke. Here "butt" means the target — the same sense as a shooting butt. "The but of the joke" is not a phrase in English.
Can I start a sentence with "but"?
Yes. It is common in edited, professional prose and has been for a very long time. Some school teachers ban it to stop pupils writing fragments, but that is a teaching device, not a rule of grammar. Use it sparingly for emphasis.