Image by freeimageslive.co.uk – gratuit

Colons are among the oldest punctuation marks in use. At first, they indicated a pause somewhat longer than a comma and shorter than a period.  Now, they more likely indicate that what follows is related in to what comes before by offering some kind of explanation. Or to put it another way, a colon means the sentence isn’t finished, and what follows is important.

Colons also have important roles besides in sentences. The rules for using them are straightforward and not controversial.

Colons in a sentence

A colon always follows a complete sentence. That is, it ought to be possible to use a period instead of a colon, if it weren’t for whatever follows the colon. It has three main uses: to introduce a list, to introduce a direct quotation, or to connect two sentences.

The last sentence of the previous paragraph illustrates using the colon to introduce a list. “It has three main uses” is a complete sentence. But a colon would be incorrect in the following instance: the colon has some other, related uses such as to emphasize the word or phrase after the colon or to introduce a question. The addition of “such as” means that what comes before the list is not a complete sentence. A colon introduces a list that comes after the end of a sentence, not within one. Bullet points, by the way, count as a list. When introducing them with a complete sentence, end it with a colon.

A colon can also introduce a direct quotation. George Bernard Shaw wrote:

I find fault with only three things in the story of yours, Jenkins: the beginning, the middle, and the end.

Both colons and semicolons connect independent clauses. With a semicolon the two clauses can have various relationships, such as causal, sequential, or oppositional, but the colon has the special function of connecting two clauses where the second somehow expands on the first.

When Bob learned that Sue had taken a job out of town, he made a firm decision: he would propose to her the next time he saw her.

Here’s another example:

LaShonda left the room in a hurry: Rod had made a racial slur, but she was in no mood to argue with him in public.

I have seen advice to copywriters to write one-line sentences and put a colon instead of a period at the end:

The colon supposedly induces the reader to continue more than a period would.

Capitalization after a colon

Note that when a colon connects two clauses, a new sentence does not start until after the period. That is, do not capitalize the first word after the colon unless, like a person’s name, it would be capitalized anyway.

That copywriter trick, however, requires a capital letter to begin the next paragraph. And here is a more interesting exception to the general rule: a colon can introduce more than one complete sentence. In that case, capitalize all of them.

Norman had a perfect plan to surprise Sally for her birthday: He rented a large room with the parking lot in back of the building. Before the guests arrived, he parked his own car on lawn in front and raised the hood. He called Sally to say that his car had broken down and asked her to come and get him. She never expected a decorated room with thirty of her friends and family members.

Why is there a colon after “birthday”? Because all the other sentences explain the plan. The colon indicates that what follows is closely related and continues the thought.

Colons not in a sentence

Use a colon between the title and subtitle of a book or article: the title of this post is an example.

If you are compiling a bibliography or making footnotes, you may or may not need colons, depending on the style guide you are following. I always use Chicago Manual of style, which expresses the volume, date, and pagination of a journal citation as “10 (2004): 45-56”. Other style guides use different formatting.

There is a difference between formal and informal correspondence. Suppose you are writing a letter to Tom Harris. (I wonder: how much longer anyone can write that without stopping to explain what a letter is?)

An informal letter could start, “Dear Tom,” with a comma at the end of the salutation. A formal letter would start, “Dear Mr. Harris:” with a colon at the end. Or, a letter from a business might start with phrases such as “To whom it may concern:” or “Dear Customer:”. Actually, I’m not sure how much the general rule colon in formal correspondence vs comma in informal correspondence matters anymore. I have in front of me a letter from one of my investments addressed to “Dear IRA Holder,”—note the comma.

Colons still come in handy in the following instances:

• cc: Tom Harris
• ATTN: Accounts Payable Department
• PS: Don’t forget to complete your paperwork before you arrive.

In expressing the time of day, use a colon to separate the hour from the minute, with no space before or after the colon: for example, 11:52.

Likewise, express a ratio as a pair of numbers separated by a colon with no spaces. The standard ratio of the width of a TV screen to its height is 4:3.

In citing scriptures, use a colon to separate chapter from verse, as in Isaiah 53:6. It is far preferable to the older method of using roman numerals for the chapter, as in Isaiah liii,6. In the same way, cite the Qu ‘ran as Qur’an 19:17.

Related posts:

The comma: punctuating or puncturing?
Semicolons: the most obscure punctuation mark
Some troubling punctuation marks