The Use of the Colon 

The colon (:) is one the least frequently used punctuation marks; nevertheless, its use is governed by certain rules and conventions. The most important feature of a colon is that it ‘looks forward’. It may be used to link two parts of a sentence where the second part describes the consequences of the first. Its other principal use is to introduce a list. These, and some other functions of the colon, are explained below. 

1.To separate two parts of sentence

A colon is used to link two parts of a sentence ONLY when the second part of the sentence is a consequence of the first. Here are some examples: 

(a)   Heavy rain is forecast for the entire day: we decided to take an umbrella with us.

(b) Heavy rain is for the entire day. We decided to take an umbrella with us.

(c)   Heavy rain is forecast for the entire day — we decided to take an umbrella with us.

(d) Heavy rain is forecast for the entire day; we decided to take an umbrella with you. 

In these examples, (a) is preferable because the decision to take an umbrella was a consequence of hearing the weather forecast. Although (b) is grammatically correct, the sentences are very short, but could be used in a novel depending on the style. The dash used in (c) is also correct, but a better use of the dash is to express an aside thought; for example, We walked through the town — it was very busy — and had a cup of coffee after visiting the shops. The semicolon used in (d) is correct, but a colon is better because it ‘points forward’ from the weather forecast to the decision to take an umbrella. 

In some sentences consisting of two or more clauses, a colon would be incorrect; for example,

I went to the office earlier than usual this morning: when I arrived there, I found that the internet connection was down.

A colon is incorrect here because the fact the internet was down was not a consequence of the writer going to the office. In this case, a semicolon would be correct,

I went to the office earlier than usual this morning; when I arrived there, I found that the internet connection was down.

It would also be acceptable to split the sentence into two or to use a dash, but in this case a semicolon is the best option to separate the clauses of the sentence. 

A colon should not be used when a link word is used to connect the two clauses; for example,

Heavy rain is forecast for the entire day: therefore we decided to take an umbrella with us.

This is incorrect, and a semicolon and a comma should be used,

Heavy rain is forecast for the entire day; therefore, we decided to take an umbrella with us. 

It should also be noted that when the colon is used to separate two clauses, the first word following the colon should not be capitalised, as the words after the colon do not form a new sentence. However, in American English in certain circumstance, the word is capitalised.  

Furthermore, the use of a colon to separate two parts of a sentence is less common in academic writing than it is in informal writing and in novels. However, if used correctly, it is acceptable in academic writing. 

2.To introduce a list

As aforementioned, the main function of a colon is to point forward. Therefore, it is particularly appropriate to introduce a list. Here are some examples: 

The University specialises in the following subjects: engineering, applied, physics, applied mathematics and computer science.  

Three departments of the university expressed an interest in asking their students to participate in a questionnaire: sociology, psychiatry, and philosophy. In this case, the colon saves the use of a link word such as ‘namely’, which could be useful if the maximum word count is critical. 

At one time it was common practice to add a dash after a colon (:–) when introducing a list. This practice is rarely used now and is considered to be outdated. A colon can also be used to introduce a list of bullet points. 

3.To introduce a question or a quotation

A colon may be used to introduce a question; for example,

As part of the questionnaire, every participant was asked: ‘How do you think your study in this subject will help you in your future career?’

A colon may also be used to introduce a direct quotation, particularly if the quotation is lengthy. 

4. To give a reference to a religious writing

A colon is used when referring to chapters and verses of religious writings; for example,       a biblical reference would be cited 1 Kings 12:6, and a reference to the Qur’an would be cited Qur’an 15:25. 

5.After a greeting in a letter

This is generally only used in American English; for example Dear Mr Smith: whereas in British English a comma is preferable. 

6.In expressing a ratio

For example, Science students outnumbered arts students by a ratio of 2:1. 

7.In expressing time

A colon is used between the hours and minutes when a particular time is quoted, such as 18:31. This is more usual in American English, but is also used in British English, particularly when the 24-hour clock is used. 

8. In reference lists

A colon is used in reference lists and in-text citations. Different referencing styles may vary and should be consulted.

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