Colons are used to introduce lists, quotes, or further explanation. They’re also used to separate items in non-grammatical structures.
A colon can be used to introduce a list. In general, the portion of sentence before the colon should be a complete sentence (it should contain a subject and a verb). Phrases like the following may sometimes be used to signal an introduction to a list. For example, “Here’s a list of groceries I need: a loaf of bread, a quart of milk, and a stick of butter.” The words preceding the colon stand as a complete, grammatically correct sentence. The list offers further explanation.
An independent clause is a group of words that contains a subject and a verb, and can stand on its own as a complete sentence. When an explanation takes the form of a second independent clause that follows a main independent clause, you can join the two clauses into a single sentence with a colon. Here is an example: “Jenny had an idea: she would pick up a cake on her way to her friend’s house.” Both the clause before the colon and the one after it are complete sentences. The clause after the colon further explains Jenny’s idea.
Similarly, a colon may introduce a quote that comes after an independent clause. For example, “Bob seemed to like that idea:‘Yeah, let’s do that!’” In this sentence, the words before the colon could stand alone as a complete sentence. The colon emphasizes the coming quote.
These uses of the colon shouldn’t be confused with the uses of the comma or semicolon. For instance, most quotes only need a comma: “John said, ‘Hello.’”
Additionally, unless the second independent clause explains the first, a semicolon is sufficient to join two closely related independent clauses. For example, “Lincoln was first elected to the Presidency in 1860; Kennedy was first elected in 1960.”
A list that isn’t introduced by a clause or doesn’t offer further explanation doesn’t need punctuation before it. For instance, “I need to visit the butcher, the baker, and the candlestick maker.”
Colons have a variety of non-grammatical uses. Titles and subtitles are separated by colons, as in William Shakespeare: The Complete Works. Colons are also found in times (12:30 pm), in ratios (10:1), in Biblical references to a chapter and verse (John 3:16), and in the salutations of business letters (Dear Mr. President:).
Colons are used to emphasize lists, quotes, or important clauses. They shouldn’t be confused with commas or semicolons, which serve different purposes.
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