6.3 Colons

Image of a colon (the punctuation mark, not the internal organ)

“Colon” is in the public domain.

Although the colon has other uses in writing, its most important function is to act as a signal to the reader—it says a definition or an example follows. Look at any standard dictionary and you will notice the formula. First comes the word to be defined; then comes the colon; then comes the definition of the word. The mark functions similarly in a sentence. A colon can also tell you a list is coming. Here are some examples.

Examples

To make a kite, you need the following items: string, paper, thin sticks, glue, and scissors. (Notice that the words before the colon make a complete statement—at least grammatically.)

The main engines of the Space Shuttle consist of six main components: the external tank, the low-pressure turbopump, the high-pressure turbopumps, the preburners, the combustion chamber, and the nozzle.

Hurricane size is expressed in four ways: the strength of the maximum winds, the diameter of the hurricane-force winds, the diameter of the gale-force winds, and the overall size of the cyclone circulation.

To make a metal dashboard, three steps are required: (1) the metal must be stamped; (2) the texture must be stamped into the metal; and (3) the part must be painted.

Notice in the last example that the first part of the sentence is actually a complete sentence itself, and it introduces a series of complete sentences. You can use the colon to connect two complete sentences—as long as the first sentence introduces or prepares for the second. Below are some examples of this.

Examples

The grades of the students in the caffeine research project told a dramatic story: the higher the caffeine intake, the lower the grades, both for semester and overall grade point average.

The line-of-sight in a communication satellite can be a problem: communication satellites can see the earth’s surface only between about 83 degrees north latitude and 83 degrees south latitude.

Many of the new applications of microcomputers are “interactive”: there is frequent interaction between the computer and one or more users.

However, don’t use a colon inside a complete sentence. It should connect only complete sentences to complete sentences or connect complete sentences to lists.

In the examples below, erroneous colons are indicated by brackets.

Examples

Problem: The typical Doppler velocity sensor consists of[:] a transistor, an antenna, and a receiver.

Revision: The typical Doppler velocity sensor consists of a transistor, an antenna, and a receiver.

Problem: Three significant types of generating plants are[:] hydroelectric, fossil-fuel-electric, and nuclear-electric.

Revision: Three significant types of generating plants are hydroelectric, fossil-fuel-electric, and nuclear-electric.

Problem: You will need[:] string, paper, thin sticks, glue, and scissors, to make a kite.

Revision: You will need the following items—string, paper, thin sticks, glue, and scissors—to make a kite.

Look at the last example closely. The grammatical core of the sentence is “You will need the following items . . . to make a kite.” You don’t want to break up the core grammar of a sentence by inserting an unnecessary colon.

Depending on the situation, consider using bullet lists to make sentences like those above easier to understand. Below are revisions using bullet lists for two of the sentences above.

Example

Three significant types of generating plants are:

  • hydroelectric
  • fossil-fuel-electric
  • nuclear-electric

Example

To make a kite, you will need:

  • string
  • paper
  • thin sticks
  • glue
  • scissors

Rules for punctuating lists vary by organization but, generally, use a grammatically correct lead-in followed by a colon. If the items in the list are individual words or phrases, use lowercase letters at the start and do not punctuate the items in the list. If the items in the list are complete sentences, capitalize and punctuate each sentence appropriately.

Example

The revised documentation process will include these changes:

  • no paper documentation
  • streamlined reporting process
  • 24/7 access to filed reports

Example

The new voting process will offer the following options:

  • Voters may go to their polling place and vote on election day.
  • Voters may register to vote by absentee ballot and mail in their votes before election day.
  • Voters may vote at their polling place at any time during the ten-day open-voting period leading up to election day.