Introduction
Colons are used to draw attention to something specific. Here are the principal uses:
1. After an independent clause and before
- a list
- an appositive (a noun or noun phrase that renames a nearby noun)
- a quotation
2. Between independent clauses if the second summarizes or explains the first.
Let’s look at each of these in detail.
Before lists
A colon is often used before a list or series:
The platypus has some unusual features: the female lays eggs (despite being a mammal), the male can defend itself with a poisonous venom, and every platypus can sense electric fields.
I have only a few things on my bucket list: visiting New Zealand, learning to quilt, and having my brain transplanted to a younger body.
It’s important to check that what comes before the colon is actually an independent clause. In the following examples, no colon is needed because the initial clause is not independent, but merges with the list to form a complete sentence:
My three favourite pizza toppings are pepperoni, pineapple, and spinach.
Her playlist includes music by “The Hot Club of Cowtown,” Johann Sebastian Bach, and “Phosphorescent.”
If you want to see some great dunks, check out videos of Dr. J, Dominique Wilkins, LeBron James, or Blake Griffin.
None of these three examples requires a colon.
Before appositives
An appositive is a noun or noun phrase that renames an earlier noun phrase. You can use a colon at the end of a sentence to introduce a concluding appositive:
Why is it that artists like Jake Bugg or “The Tallest Man on Earth” imitate Bob Dylan’s worst feature: nasal singing?
You know there’s one thing I could really use this winter: a remote car-starter.
In the first example, “nasal singing” specifies what the “worst feature” is; in the second example, “a remote car-starter” clarifies what is meant by the “one thing.”
Make sure, though, that you have an independent clause before the colon. That’s why the following example is incorrect:
Incorrect: The title of his dissertation was: “In Every Orifice: The Origins of the Thermometer.”
There is no need for a colon here because the introductory clause cannot stand by itself as a complete sentence.
Before quotations
Quotations are normally preceded by a comma or a colon, if they are not simply merged with the writer’s own words. Compare the following examples:
Bernard called the accusations “preposterous.”
Stephen Leacock once quipped, “Many a man in love with a dimple makes the mistake of marrying the whole girl.”
The minister suggested the following text for our wedding: “‘I am against you,’ declares the LORD Almighty. ‘I will lift your skirts over your face. I will show the nations your nakedness and the kingdoms your shame’” (Nahum 3:5).
As the last example shows, you can put a colon before the quotation if you’ve introduced it with an independent clause.
Between independent clauses
If you want to join together two independent clauses without using a conjunction, the most common method is to use a semi-colon. However, if the second clause is more specific than the first, or if it explains what came before, then you may want to use a colon:
Swimmers and rock climbers like to know whether their arm span is longer than their body length: This measurement is called the “ape index.”
The Bogomils were Gnostic dualists: they believed that all earthly matter was evil, but that the spirit or soul was divine.
Please note that the independent clause after the colon may be capitalized (e.g., compare This and they in the examples). Whatever you choose to do, make sure you’re consistent.
Minor uses
Finally, you will have noticed that a colon is also used in time measurements, ratios, citations of biblical passages (and some other texts), and in titles:
He struggled to memorize John 11:35.
At 12:55 a.m., I finished my thesis paper, titled “Watt is Wrong With You?: Decreasing Abilities in Light Bulb Installation.”