Grammar: The Colon [ : ]

 

Announcing Information

One of the most common uses of the colon is to announce information that elaborates something that has gone before, often a rule, moral or 
statement of principle. In this capacity, the colon acts as a substitute for that, that is, that is to say or namely 

      New employees are told that there is only one rule: the customer is
      always right.

      His lifelong career of unpunished crime illustrates the unfortunate truth:
      you can have your cake and eat it.

(Note that the single dash may informally be used instead of the colon.)

As a substitute for these words, it follows that the colon should not be used with them 

      New employees are told that there is only one rule: namely, the customer
      is always right.
(Incorrect)

      His lifelong career of unpunished crime illustrates the unfortunate truth:
      that is, you can have 
your cake and eat it. (Incorrect)

If we want to use namely or that is, we omit the colon 

      New employees are told that there is only one rule, that is, the customer
      is always right.

 

Preceding Lists

A related function of the colon is to precede lists, whether horizontal 

      Many things need to be considered before starting your own business:
      premises, finance, demand for the product, staffing and access to
      customers

or vertical 

      Many things need to be considered before starting your own business:

              premises
            •  finance
            •  demand for the product
            •  staffing
            •  access to customers

As explained in lists, however, a colon should not be used to precede lists whose introductory statements cannot be construed as complete
sentences, 
that is to say, where the listed items continue the sentence 

      Among the many things that need to be considered before starting your
      own business are

            •  premises
            •  finance
            •  demand for the product
            •  staffing
            •  access to customers

(In the vertical list, note that there is no and between the last two items and no full stop at the end.)

 

Announcing Quotations

The colon can also be used to announce quotations, but only when preceded by an independent clause (one that can be read as a complete
sentence) 

      When asked if it was easy to start her own business, she laughed: You
      must be joking! (Correct)

      When asked if it was easy to start her own business, she said: You must
      be joking! (Commonly encountered but incorrect)1

The colon in the second example should be replaced with either a comma 

      When asked if it was easy to start her own business, she said, You must be joking!

or nothing at all 

      When asked if it was easy to start her own business, she said You must be joking!

 

Separating Numbers

The final use of the colon is to separate the numbers of chapters and verses, especially in the Bible 

      Corinthians 13:11

and, in American English, to separate the numbers on a clock 

      The train departs at 3:30 PM

Digital clocks universally adopt the American practice but, elsewhere, British English uses a point 

      The train departs at 3.30 p.m.

Note, too, the differences between the American AM / PM and the British a.m. / p.m. (or am / pm).

 

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1 The Guardian Style Guide contradicts this: ‘Use [a colon] before quotes when the quote could stand on its own as a sentence. He said: You'll
   never take me alive.