Use a colon when what follows the colon amplifies or further explains what precedes it. For example:
Rachel had just two goals that summer: to learn French and to pass the required swimming test.
In the sentence above, what appears before the colon — two goals that summer — is equivalent to what appears after it (to learn French and to pass the required swimming test). The colon adds dramatic impact by creating suspense; the reader must wait until the end of the sentence to find out what Rachel’s goals were.
In part because the colon can be high-impact, you should use it sparingly. In this next example, note how the colon adds punch by deferring the central revelation of the sentence:
The committee concluded that responsibility for the scandal rested with just one person: the CEO.
In many cases, the colon sets up a kind of equivalency. For instance, if you look just to the left of the colon in the example above, you see the words just one person. Look just to the right of the colon and you see that person identified. If this were a math problem, you could put an equals sign where the colon stands.
The same goes for the sentence below:
The company introduced several new products: lawn chairs, beach blankets, and water-resistant radios.
The several new products are equal to the lawn chairs, beach blankets, and water-resistant radios. But what if we added the word including after products? The following sentence illustrates a misuse of the colon, one that is very common in business writing:
The company introduced several new products, including: lawn chairs, beach blankets, and water-resistant radios.
The presence of a list is not by itself enough to justify a colon. For one thing, we no longer have roughly equivalent ideas to the left and right of the colon. As a general rule, do not use a colon after a form of the verb include. In the incorrect sentence above, no colon is necessary; it should be deleted.
The company introduced several new products, including lawn chairs, beach blankets, and water-resistant radios.
An important exception would occur if we turned the list of new products into a bulleted list. In this case, the colon would be reinstated, as shown here:
The company introduced several new products, including:
- lawn chairs
- beach blankets
- water-resistant radios