A couple days ago I posted on semicolons, with a note that I’d decided to split my original post, covering both semicolons and colons, into two parts once I realized that the semicolon discussion was getting out of control.  Okay, so now for colons.  These are a lot more fun than semicolons 🙂

The first thing to know about the colon is that it is a mark of “enumeration.”  To “enumerate” is to count off all the specific items in a collection.  Hence the “numer” in the term, as in the word “numeral.”  To continue with the math metaphor, I also think of a colon as being similar to an “equals” sign.  Whatever comes after the colon equals whatever preceded it, but in more specific detail.

There are two main times you should use colons to punctuate sentences.  The first is to introduce a list/quote/idea, and the second is to join two independent clauses with an appositional relationship.

Using a colon to introduce a list, quote, or idea — You already no doubt have used the colon with lists.  But what you may not know is that you need to precede the colon with the THING (a noun or noun phrase) that is subsequently being listed.  A common colon error is to put the colon in front of the list but after a verb.  Think about it, though.  If the colon is an equals sign, then how can a verb = nouns or adjectives?  Here is a list of nouns.

WRONG: We need: milk, butter, and eggs.

RIGHT: We need the following ingredients: milk, butter, and eggs.

RIGHT: We need the following ingredients to make cookies: milk, butter, and eggs.

The THING we need is “ingredients.”  “Ingredients” is a noun, as are “milk,” “butter,” and “eggs.”

Here is colon preceding a list of adjectives.

WRONG: He was: cold, wet, and hungry.

RIGHT: He was miserable: cold, wet, and hungry.

“Miserable” is an adjective, and so are “cold,” “wet,” and “hungry.”

You also no doubt know about using a colon to introduce a quote or idea.  I’ve been doing it with the words “WRONG” and “RIGHT” in this blog post.  First comes the word “WRONG,” then the colon, and then the specific quote/idea that is wrong.  So “WRONG” = this sentence.

With dialogue or quoted statements within a text, a speaker’s words are usually introduced with a comma.  However, you can use a colon for emphasis if you like.  And a colon would always be used if you preceded the person’s exact words with a noun (or noun phrase), as in the third sentence below.

The President said, “The only thing we have to fear is fear itself.”

The President said: “The only thing we have to fear is fear itself.”

One phrase in President Roosevelt’s inaugural address stands out memorably: “The only thing we have to fear is fear itself.”

Using a colon to join independent clauses with an appositional relationship — A colon is sort of like a prism: plain light goes in one side, and all the colors included in its wavelength come out the other.  In other words, the colon refracts general information into its more specific iteration.

If you think about the colon as an equals sign or a prism, then it’s a cinch using it to join two independent clauses correctly.  The first independent clause is the general statement, and the second clause IS the specific enumeration/iteration/naming of that idea.

One thing is clear: Mary is well qualified to be president.

Or, the first clause is the general statement, and the second clause EXPLAINS that statement.

Everyone wants Mary to be elected: she is the most qualified candidate.

Should you capitalize the first letter of what follows a colon or not?  This is a usage issue, and experts/style guides don’t agree on what is “correct.”  My own practice is to use the lower case unless I want to highlight/emphasize the element that follows the colon.  I also use one space to follow a colon, unless I’m capitalizing the first word that follows it.  Then I use two spaces.

Using the colon correctly is like learning to ride a bike.  Once you get the concept, you really can’t make a mistake.  Plus, it’s a “smart” mark of punctuation:  Using it well elevates your writing style a notch.