Capital has many definitions. It can mean “the wealth owned or employed in business by an individual, firm, corporation, etc.”; it can mean “principal; highly important,” as in Safety was their capital concern; and it can mean “uppercase letter.” But the capital that gives most people trouble is this one: “the city or town that is the official seat of government in a country or state,” as in The capital of California is Sacramento or The capital of the United States is Washington, DC.
Capitol also has to do with government, but it is more specific: capitol is defined as “a building occupied by a state legislature.” When the word Capitol is capitalized, it refers to the United States Capitol, a building in Washington, DC, that hosts Congress, the legislative branch of the US federal government.
Both capital and capitol are derived from the Latin root caput, meaning “head.” Capital evolved from the words capitālis, “of the head,” and capitāle, “wealth.” Capitol comes from Capitōlium, the name of a temple (dedicated to Jupiter, the Roman equivalent of the Greek god Zeus) that once sat on the smallest of Rome’s seven hills, Capitoline Hill.
Because capital and capitol are so close in spelling, it can be challenging to remember which word denotes what. As a mnemonic trick, consider the importance of the letter “o” in capitol. This “o” stands for “only one” definition, while the “a” in capital stands for “all the rest.” If that doesn’t work, try associating the “o” with the spherical dome of the US Capitol building.
http://blog.dictionary.com/capital-vs-capitol/
