Definition: A colon is a punctuation mark that looks like a period above a period (:). It generally introduces a list, a definition, a clarification, or additional details. It can also be used to emphasize, highlight, or draw attention to what follows it. The statement that precedes a colon is always an independent clause, but what follows the colon can be either a dependent or independent clause. (This is in stark contrast to the semicolon, which always connects two independent clauses.)
Introducing a Series
Colons are used after an independent clause that introduces a list or series.
Please buy these items: a screwdriver, a hammer, a measuring tape, and some nails.
Please proofread for the following: periods, commas, and colons.
Some people think that a colon must precede all lists, but this is not true: a colon can only be used after an independent clause. In the following sentences, you might be tempted to put a colon after your or got, but this isn't correct because the clause preceding the list is not an independent clause:
When you leave, don't forget to bring home your shoes, bathing suit, and towel.
These stickers are for those who got As, those who did extra credit, and those who turned their projects in early.
For dramatic effect, authors sometimes reverse the order of the sentence by placing the series before the colon and the independent clause after the colon.
Red, blue, and white: the decorations for the party were very patriotic.
Appositives After Independent Clauses
Colons are used before appositives that follow an independent clause. A comma could also be used, but a colon is stronger. Use a colon instead of a comma when you want to emphasize or call special attention to the appositive.
I only like two flavors of ice cream: chocolate and vanilla.
We visited a famous World Heritage Site: Stonehenge.
Colons are used to formally introduce quotations. The formal introduction must be an independent clause. This is a great way to add dramatic effect to a quotation, but use it sparingly.
Whenever we helped my mother in the kitchen, she always quoted her grandmother's favorite aphorism: "Many hands make light work."
The coach's words stayed with his team: "I just want you to play your best."
Summaries or Explanations
A colon can be used to combine two independent clauses if the second clause summarizes or clarifies the first. The second sentence can begin with either a lowercase or capital letter.
I got rained on the entire walk: I am soaked.
This is what I wrote on the board: Homework due tomorrow!
After the greeting in a formal or business letter
Dear Sir or Madam:
I would like to address a recent issue with your company …
In scripts to indicate dialogue
Sometimes you can express dialogue by using a name followed by a colon. Quotation marks are not necessary with this style, which is typically used in scripts. With this style, the characters' names appear in all capitals.
MACI: I missed the party because I was sick.
JACKIE: I'm sorry to hear that. You missed a great party!
Between hours and minutes in time
We will leave at 5:30 this evening.
Between numbers to show ratios or proportions
The best proportion of students to teachers is 20:1.
Between titles and subtitles of works
My favorite television show then was Star Trek: The Next Generation.
Between city and publisher in bibliographic entries
New York: Best Books, 2012
Between chapter and verse in biblical references
Grandma always quoted John 3:16.
Some style manuals recommend a period instead.
Grandma always quoted John 3.16.
Caution: Slippery when wet
Note: Use colons correctly
To: My Boss