Colon (punctuation)

A colon (:) signals that what follows will explain, illustrate, or itemize what came before. It can introduce a list ("Pack three things: water, sunscreen, and a map"), an explanation ("She had one goal: victory"), or a quotation. The essential rule is that the words before the colon must form a complete sentence — an independent clause that could stand on its own.

That rule catches the most common colon errors. A colon should not interrupt a verb and its object ("My favorite subjects are: math and history" is wrong because "My favorite subjects are" is not a complete sentence) and should not follow phrases like "such as" or "including." Remove the colon in those cases, or rewrite so a full sentence precedes it: "I have two favorite subjects: math and history."

Do you capitalize after a colon? Always capitalize a proper noun. Beyond that, style guides differ: what follows a colon is usually lowercase when it's a fragment or list, while some styles capitalize when a complete sentence follows. Standardized tests rarely hinge on this — they test whether the colon itself is used legally. It also helps to know the colon's neighbors: a semicolon joins two related independent clauses, while a colon requires a complete sentence only on the left and points forward to what follows.

Colon usage is tested directly on the ACT English section, which loves to check whether an independent clause precedes the colon and whether the colon is separating a verb from its object. Achievable's ACT course covers colons and semicolons side by side so you can tell at a glance which mark a sentence needs.