The colon acts like a gate. After the gate, additional information is provided to explain something about the first part. In this section, we’ll look into how to use the colon (:) to (1) join two sentences by their mutual content and (2) introduce a bulleted form of points or list items. Finally, we’ll (3) highlight when to use capital letters following a colon.

Colon use 1/3: To join two sentences by mutual content

After a complete sentence to replace namely, for example, and such as.

The main rule behind this usage of the colon is ensuring the part before the colon sounds like a complete sentence. Imagine the first part before the colon: if it sounds complete, you have used the colon correctly.

Try reading aloud the below examples, comparing the correct with the incorrect ones, and stopping when you reach the colon. Does it sound like you just read a complete sentence?

Correct exampleRead the part before the colonIncorrect example
Two possible strategies exist: individuals either continue to invest in development or cease reproduction.

Two possible strategies exist.

Yes, sounds like a complete sentence.

Two possible strategies are: individuals either continue to invest in development or cease reproduction.

Incorrect because the terms before the colon form an incomplete sentence.

The model incorporated the following variables: age, size, and weight.

The model incorporated the following variables.

Yes, sounds like a complete sentence.

The model incorporated the variables: age, size, and weight.

Incorrect because the part before the colon does not adequately introduce the second part.

Our study tested the following conditions: whether the experimental 30:70 P:C group showed different growth responses than the 50:50 P:C control group.

Our study tested the following conditions.

Yes, sounds like a complete sentence.

Our study: tested whether the experimental 30:70 P:C group showed different growth responses than the 50:50 P:C control group.

Incorrect because the terms before the colon form an incomplete sentence.

We also use colons to join two, closely related sentences when there is emphasis on the second sentence, i.e. the second sentence explains the first. Here, there is no need to use a coordinating conjunction, i.e. therefore, conversely, similarly.

Correct exampleIncorrect example
The artificial diet showed a positive growth effect: individuals gained more weight and survived better.The artificial diet showed a positive growth effect, individuals gained more weight and survived better. Incorrect because uses a comma splice and creates a run-on sentence.
The bleached samples were statistically significant: P values ranged between 0.02 and 0.001. A colon is preferred over a semi colon because the second sentence is the emphasis and explains the first sentence.The bleached samples were statistically significant, P values ranged between 0.02 and 0.001. Incorrect because uses a comma splice and creates a run-on sentence.

Colon use 2/3: To introduce a bulleted form of points or list items

When preceding bulleted form, especially when the bullet points contain lengthy or multiple sentences. This method is sometimes optional as presenting a bullet form depends on the position of emphasis in the paragraph. For example, when closing off a paragraph and placing the position of emphasis at the end of the paragraph, a period may be sufficient. In contrast, and more commonly, bulleted lists typically introduce a set of ideas or standalone points that invite extra consideration from the reader. Therefore, the sentence preceding the bulleted list should be introductory and thus end in a colon.

Correct exampleIncorrect example

Our study addressed the following aims:

1. Aim 1

2. Aim 2

3. Aim 3

Our study addressed the following aims.

1. Aim 1

2. Aim 2

3. Aim 3

Incorrect because the sentence is introductory and emphasizes a point.

Colon use 3/3: When to use capital letters following a colon

Use a capital letter when there are two or more sentences after the colon when the sentences are directly related:

Correct exampleIncorrect example
Reading this sentence provokes further thinking: There is an immediate connection to the subject. There is also some connection with the presenter.

Reading this sentence provokes further thinking: there is an immediate connection to the subject. There is also some connection with the presenter.

Incorrect because the second and third sentences are directly related to the first.

Our PCA analysis showed conflicting results to previous work: These results also conflicted with those of Clarke (2002). Swanson (2006) previously confirmed this disparity.

Our PCA analysis showed conflicting results to previous work—these results also conflicted with those of Clarke (2002). Swanson (2006) previously confirmed this disparity.

Incorrect because an em dash is used to express shift in tone but there is no shift in this example sentence.

Use a capital letter when the sentence following the colon is a short introductory phrase.

Correct exampleIncorrect example
The results were surprising: Further analysis is warranted. The results were surprising and further analysis is warranted.The results were surprising: further analysis is warranted. Incorrect because the first sentence is introductory and wishes to emphasize an important point in the second sentence.

Use a capital letter when the first sentence is introductory and the second sentence expresses a rule:

Correct exampleIncorrect example

Consider for a moment the following point: A colon should only follow a complete sentence.

Here the colon clarifies that the rule is just the next sentence, and nothing after that.

Consider for a moment the following point. A colon should only follow a complete sentence.

Incorrect because the second sentence is a follow-on rule from the first introductory sentence.

Use a capital letter to introduce a quotation:

Correct exampleIncorrect example
We based our choice of statistical tests on Jones et al.’s (2010) who stated the following: ‘As a basic statistical assumption, non-parametric data should first be transformed’.

We based our choice of statistical tests on Jones et al.’s (2010) who stated the following. ‘As a basic statistical assumption, non-parametric data should first be transformed’.

Incorrect because a period does not adequately introduce a follow-on quotation.