Colon - a punctuation mark used to build upon whatever comes before it.
A colon is an indication to the reader that any upcoming information is closely related to the previous clause.
It shares similarities to the semicolon but is distinct in its usage.
A colon can be used to introduce a list of items; however, the preceding clause must be an independent clause.
Example
There are three ingredients in a cake: flour, eggs and sugar.
Note that the following example is incorrect because the clause before the colon is not independent.
Incorrect
The three ingredients in a cake are: flour, eggs and sugar.
A colon can be used to introduce a quotation, but only if the clause before the colon and/or the quotation is independent.
Preceding clause is independent
A bystander commented on the recent fire: "Just horrible!"
Quotation is independent
The mayor stated: "This was a disaster that should never have happened."
The following example, in which both the preceding clause and quotation are dependent clauses, is incorrect.
Incorrect
The mayor exclaimed: "Just horrible!"
In this case, a comma should be used before the quotation.
Correct
The mayor exclaimed, "Just horrible!"
The trickiest application of the colon, this rule allows a writer to expand upon an independent clause to help the reader better understand or appreciate this clause.
The clause which follows the colon in this instance must also be independent, and it must have a direct application to the preceding clause (unlike the semicolon, which does not require such a direct link).
Examples
I cannot live without my phone: notifications dictate every aspect of my life.
Reading for pleasure helps to improve wellbeing: studies show that it benefits sleep patterns and positivity.
Note that in British/Australian English, a capital letter is not required after a colon. In US English, however, some guides do recommend capitalisation.
A colon can be used for dramatic effect when tension is generated in the preceding independent clause.
The information that follows the colon should conclude the thought that is being expressed in the sentence, although it does not have to be an independent clause itself.
Examples
There was only one thought on her mind: revenge.
After six months of tense proceedings the jury finally gave its verdict: not guilty.
A colon can be used to write time like a digital clock. No space should be left before or after the colon in this case.
Example
3:15 pm
In Australian English it is acceptable to use a full stop in place of the colon, and no full stops are required when writing a.m. or p.m.
A colon can be used to demonstrate a ratio between two numbers. No space should be left before or after the colon.
Example
1:10
When referencing chapters and verses from the bible, use a colon to separate the two (with no space on either side).
Example
Ezekiel 25:17
A colon can be used in a number of ways in both personal and business correspondence.
Examples
Dear Mr. Citizen:
cc: Jane Smith
Attention: Customer Service Department
PS: Don't forget to send photos!