Capitol, spelt with an o, refers only to a building where a legislature meets. Capital, with an a, covers every other meaning: the leading city, money, an uppercase letter and the adjective in capital punishment. Protesters gathered outside the Capitol. Ottawa is the capital of Canada.
The core difference
Capitol is the narrow one. It names a building — specifically the building where a legislature sits — and in American usage it is normally capitalised: the Capitol in Washington, DC, or a state capitol in Austin or Sacramento. That is essentially its only meaning.
Capital is the broad one, and does at least four separate jobs:
- A city: Lisbon is the capital of Portugal.
- Money or assets: The business needs more capital before it can expand.
- An uppercase letter: Names take a capital letter.
- An adjective meaning punishable by death, or of prime importance: capital punishment, a capital offence.
The test you can run
The Capitol has a dome — and both have an O. If you are pointing at a building, use the o. In every other sentence, use capital with an a.
A second check: if the word could be replaced by "main", "chief", "funds" or the name of a city, it is capital. If it could be replaced by "that building over there", it is capitol.
How they get tangled
The commonest error is writing "the state capitol" when you mean the state's leading city, or "the nation's capitol" when you mean Washington as a city. Washington is the capital; the Capitol is one building in it. Similarly, Capitol Hill is the district around the building, so it takes the o — which is why "Capital Hill" is such a frequent slip in headlines.
An honest note on where you need this
Outside the United States you will almost never need capitol. British, Irish, Australian and Canadian legislatures meet in a Parliament, a Senedd, an Oireachtas or a Parliament Hill — none of them a capitol. If you are writing for a non-American audience and reach for capitol, check that you are not simply misspelling capital.
The one exception you will meet abroad is in names of cinemas, theatres and hotels — the Capitol Theatre and similar — which borrow the same spelling as a grand-sounding proper noun.
Related words that share the root
Capitalise, capitalism, capitalist and per capita all take the a spelling. There is no capitolise. And note the two senses of capitalise: to write with an initial uppercase letter, and to provide funding for a business — plus the phrase capitalise on, meaning to take advantage of.
British and American spellings are identical here; only the meanings differ, so a spellchecker will never catch the wrong one for you.
Frequently Asked Questions
Is it Capitol Hill or Capital Hill?
Capitol Hill, with an o. It is named after the Capitol building that stands on it, so the building spelling carries over to the district and to the phrase used for Congress generally.
Is Washington, DC the capital or the capitol?
The capital. Washington is the capital city of the United States; the Capitol is the specific building on Capitol Hill where Congress meets.
Do British writers ever need the spelling capitol?
Rarely. There is no British capitol building, so outside American contexts the word turns up mainly in proper names such as the Capitol Theatre. In most British writing, <em>capital</em> is the word you want.