Colons (:) are used to indicate the beginning of a list. Use two spaces after a colon.

This experiment requires the following: a function generator, an oscilloscope, an RC circuit, and an inductor.

You could present the same information in the following way.

This experiment requires a function generator, an oscilloscope, an RC circuit, and an inductor.

In the latter example, no colon was used. In the former, I used a colon as a way of saying "Attention! Here comes the list!" The information after the colon is specific. The phrase before the colon prepares you for the list. The latter example has the same information content as the former. There is more emphasis on what the list items are if a colon is used.

Last week, I said

"Do not use commas to join two independent clauses; semicolons (;) or periods are preferred.

Stevenson's romances are entertaining; they are full of exciting adventures.
Stevenson's romances are entertaining. They are full of exciting adventures."

There is one other situation in which to use semicolons. If you have a long list where each item is composed of several different things, then separate the items with semicolons.

On my vacation I shall take: two pairs of skis and boots, each optimized for different snow conditions; warm clothing for cold weather and lighter clothing for sunny days, along with my favorite mult-purpose hat; and good reading material, including my favorite book, which I've already read three times.

Scientists frequently use colons and semicolons in figure captions.

Figure 1. The major results of our study: a) TEM image of bacteria; b) AFM image of bacteria; c) cross-sectional profile of the bacteria; and d) corresponding response as a function of load.

Punctuate the following using commas, semicolons, or colons. Optional punctuation is indicated by parentheses.

1. Eat, and drink this tea.
2. Listen, and write an essay.
3. Do the experiment(,) and write a paper.
4. Do the experiment; write a paper.
5. We used the following approach:  we first laid the DNA on the substrate and let it dry; then we used an AFM to image the DNA and determine its conformation; the image was then used to generate data; and(,) finally(,) the Flory exponent was determined.
6. I need three items:  methanol, acetone, and distilled water.

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Created 30 April 1998, by Nancy Burnham and Fred Hutson.