That A Colon
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Colons with Formal Appositives
Use a colon instead of a comma to introduce an appositive at the end of a sentence for emphasis.
Appositives may be words, phrases, or clauses. If it is an independent clause, that clause begins with a capital letter.
Correct: He was watching his favorite type of television show: a baseball game. (A comma is fine, but a colon here provides emphasis and/or formality.)Correct: He learned a valuable lesson: Never argue with a woman. (The appositive here is an independent clause, so it is capitalized.)