Introducing...the colon

If you’ve ever had trouble using the colon in your text, you are not alone. I often encounter mistakes in colon use during copy editing and proofreading. This post contains information on how to properly use the colon to polish and clarify your writing.

Here I divide the job of the colon into two categories: formatting and introducing.

Colon Use for Formatting

Most writers are familiar or confident when using the colon to format. Colon use for formatting can be dependent on your style guide or field, so make sure you always check the guide or field conventions. Here’s a list of how the colon is commonly used in formatting:
Time

1. 3:45 pm (time of day)
2. 04:25:02 (hrs:min:sec)

Ratios

1. 1:1 (a one-to-one mixture)
2. v:v (a volume-to-volume mixture)

Formal address
    Dear Dr Smith:
Books of the Bible

1. Matthew 1:3
2. Genesis 2:15

Below are examples of colon use in two different academic fields.

The American Medical Association uses the colon in references between the issue number and page numbers.
Author(s), Article title. Abbreviated Journal Name. Year;vol(issue No.):inclusive pages. DOI

The Council of Science Editors uses the colon in titles and subtitles (with the caveat that the period can also be used—check the style guide!)

Scientific  Style and Format: The CSE Manual for Authors, Editors, and Publishers

Always double check the conventions in your field when you use the colon for formatting!

Colon Use for Introducing Ideas

Like the semicolon and comma, the colon indicates a strong pause within the sentence. But unlike the semicolon and comma, where the pause indicates two closely related ideas, the colon introduces the information that follows. The best way to think of it is that the information after the pause illustrates, expands, or clarifies the statement before the colon. This is key to using the colon correctly in your writing. Once you effectively master colon usage, your writing will have more finesse and polish.

Using the Colon to Introduce Lists

“We used three techniques to characterize the product: NMR spectroscopy, mass spectrometry, and elemental analysis.”

The above example has a complete independent clause before the colon; the list of items following the colon clarifies the idea by listing the techniques used. To check if this is correct, separate the information into more than one sentence.

“We used three techniques to characterize the product. They were NMR spectroscopy, mass spectrometry, and elemental analysis.”

Although both versions are correct, the colon can combine these sentences into one, thereby improving the flow of the text.

Here’s an incorrect version that has an incomplete clause before the colon, where a preposition is separated from its direct objects by a colon.

“We characterized the product by: NMR spectroscopy, mass spectrometry, and elemental analysis.”

If you check this with a period you get, “We characterized the product by.” This is an incomplete sentence! Wrong!

The use of colons to enumerate lists can be tricky when the words “including” and “the following” are used.

Incorrect: “We used multiple techniques to characterize the product, including: NMR spectroscopy, mass spectrometry, and elemental analysis.”

Correct: “We used multiple techniques to characterize the product, including NMR spectroscopy, mass spectrometry, and elemental analysis.”

The colon is not used when “including” is present even though it indicates a list to follow. But, the colon is used when “the following” or “as follows” is present.

“We characterized the product with the following techniques: NMR spectroscopy, mass spectrometry, and elemental analysis.”

The colon can also be used for lists that aren’t in-line with the text (as I showed above). The next time you need to present an in-line list, try using a colon!

Using Colons to Introduce Ideas

So far, I’ve given examples where I use a colon to introduce an in-line list or series to clarify the idea before the colon, but the colon can introduce a phrase, a complete sentence, a long quote, or, as in the sciences, equations.

“Pressure, volume, and temperature are related by the ideal gas law: PV = nRT”

“Einstein’s most famous equation relates energy, mass, and the speed of light:
E = mc2,
where E = energy, m = mass, and c is the speed of light.”

Notice that the punctuation in the dependent clause after the colon follows normal grammar rules.

Here’s an example where the colon introduces another sentence.

“The colon is a powerful punctuation mark: a variety of informative statements can be introduced by the colon.”

Rules Governing the Colon

Capitalization

Making the initial letter after the colon upper or lower case will ultimately be determined by the style guide you are using and conventions of your field, but here are some basic tips.

For items in a list, if the listed items are themselves complete sentences, use a capital letter after the colon and for each item. If the listed items are not complete sentences, make the initial letter lowercase. However, if there is only one sentence after the colon, you may need to keep the initial letter lowercase—check your style guide!

Commas, semicolons, periods, or numbers?

Items presented in an in-line list can be separated by commas, semicolons, or periods. Use periods for sentences; semicolons when the item has commas within it; and commas for non-sentences. Whichever you choose, the punctuation must be consistent within the list.

For displayed lists, semicolons and commas aren’t used between items. Periods are only used when the items themselves are sentences. Closing punctuation is not always required after the final item in a displayed list—again, check your style guide!

Numbers, enclosed by parentheses, may be used. This will depend on how you want to present the information. For example, if you are writing a grant application and a certain number of specific aims are required, you may want to number these aims in your list. Check the author guidelines!

Learn to Use the Colon!

Just like any other skill, using the colon correctly in writing takes practice. When used correctly, the colon can improve the precision, clarity, and flow of your text.

How do you feel about using the colon? Leave a comment and we can chat!