Editing Tip: Colon Usage

The colon (:) is a punctuation mark used for illustration or emphasis. Unlike the semicolon, colons represent a closer link between the two parts of a sentence and require only the first half of the sentence to be an independent clause. You are most likely familiar with the use of a colon to introduce a list of specific examples:

• Three supplements were added to the cell medium: fetal bovine serum, penicillin, and streptomycin.

To facilitate the flow of a list after a colon, an additional term is often used, such as following or as follows:

• The following supplements were added to the cell medium: fetal bovine serum, penicillin, and streptomycin.

Again, keep in mind that the phrase before the colon must be an independent clause. A colon thus cannot be placed between a verb and its object or a preposition and its object, as in the examples that follow:

• The supplements added to the cell medium were: fetal bovine serum, penicillin, and streptomycin. (Incorrect)
• The cell medium was supplemented with: fetal bovine serum, penicillin, and streptomycin. (Incorrect)

A colon may also be used to introduce a dependent clause that helps to emphasize or illustrate the idea in the preceding clause:

• There is only one way to culture that cell line: in medium with L-glutamine.

Note that in both of the above cases (colon use before a series or before an illustrative dependent clause), an em dash (—) can be included instead of a colon in more informal writing. A lesser-known application of the colon is the connection of two independent clauses. As discussed above, this punctuation should only be used between two closely related ideas, as in the case of illustration:

• The experiment using that cell line was successful: we finally proved that the cells require cytokines to proliferate.
• In the presence of cytokines, the cells proliferated: in the absence of cytokines, the cells grew poorly. (Incorrect; requires semicolon)

Although a period between the two independent clauses would be grammatically correct, a colon helps the reader to understand the logic of the ideas. We hope that today's editing tip has taught you how to use colons to improve the clarity and flow of your writing. If you have any questions about this valuable punctuation mark, please email us at [email protected].

Modified from version originally published on AJE's Expert Edge blog.