A colon introduces a further thought, one added to throw light on a first. Some writers use a capital letter to start any complete sentence that follows a colon; others prefer a lowercase letter. Whichever you choose, be consistent. A phrase that follows a colon always begins with a lowercase letter.

Like a semicolon, a colon can join two sentences into one. The chief difference is this: a semicolon says merely that two main clauses are related; a colon, like an abbreviation for that is or for example, says that the second clause gives an example or explanation of the point in the first clause.

She tried everything: she scoured the Internet, made dozens of phone calls, wrote e-mails, even consulted a lawyer.

A colon can introduce a word, a phrase, a series, or a second main clause, sometimes strengthened by as follows or the following.

The dance steps are as follows: forward, back, turn, and glide.

When a colon introduces a series of words or phrases, it often means such as or for instance. A list of examples after a colon need not include and before the last item unless all possible examples have been stated.

On a Saturday night many kinds of people crowd our downtown area: drifters, bored senior citizens, college students out for a good time.

A colon preceded by a main clause can introduce an appositive.

I have discovered the key to the future: robots.

Sometimes you can’t conveniently introduce a quoted passage with a comma. Perhaps the quotation is too long or heavily punctuated, or your prefatory remarks demand a longer pause. In either case, use a colon.

God told Adam and Eve: “Be fruitful, and multiply, and replenish the earth, and subdue it.”

In a sentence, a colon always follows a complete sentence, never a phrase. Avoid using a colon between a verb and its object, between a preposition and its object, and before a list introduced by such as.