Colons :

The colon has 3 main functions as a punctuation mark:

  • separating the main part of a sentence from secondary or extra content
  • dividing the title and subtitle of publications and in headings
  • formatting mathematical ratios.
Lists
Introduce a vertical list or a complex serial list
We distribute the magazine to:
  • newsagents
  • bookshops
  • individual subscribers.

The most common use of a colon today is at the end of the introductory sentence to a vertical list, whether the following points are full sentences or fragments.

You are eligible if you meet the following criteria:

  • You have been an Australian resident for at least 5 years.
  • Your annual income is below our threshold of $80,000.
  • You will lodge an individual income tax return this financial year.
For short serial (in-sentence) lists, you won't need a colon.
Our catering includes wraps, salads, yoghurts and fruit.

However, if an in-sentence list is more complex, you can add a colon after the introductory clause and separate the list items with semicolons.

Our catering includes: wraps filled with chicken, beef or vegetarian options; mixed-leaf salads; individually portioned yoghurt pots; and a selection of seasonal fruit.
If you find yourself doing this, consider running the list vertically down the page with bullets for ease of reading.

Our catering includes:

  • wraps filled with chicken, beef or vegetarian options
  • mixed-leaf salads
  • individually portioned yoghurt pots
  • a selection of seasonal fruit.
Amplification or explanations
Introduce a second independent clause that amplifies, explains or contrasts with the first
The question is simple: what criteria shall we use?
Our review revealed the opposite: the system was to blame.

The second most common use of a colon is to separate 2 parts of a sentence, where the second part adds to or comments on the first.

This works well as a form of emphasis by drawing attention to some addition or qualification about a statement.

High temperatures and low humidity can only mean one thing: bushfire season has begun.

The amplification function of a colon operates similarly to that of a dash – although a dash adds slightly stronger emphasis. The other major difference is that a pair of dashes can emphasise an additional element in the middle of a sentence, whereas a colon can only amplify at the end.

Block quotes
Introduce a quote that is set apart from the body text as a block or paragraph
The complainant alleged that:
The report made the following recommendations:

If you need to quote a short piece of text (1 to 2 lines) from another source, the simplest option is to put it into quote marks within the same sentence.

Where you need to cite a longer piece of text, it is best to do so in its own paragraph (usually indented) and introduce it by ending the previous paragraph with a colon. You can also do this with a short piece of text if you want to emphasise the quote and it is at least 1 line long.

Matthew Arnold once wrote about style:

People think that I can teach them style. What stuff it all is. Have something to say and say it as clearly as you can. That is the only secret of style.

Note that when you quote a block of text (particularly if you indent it) you do not also need to put it in italics or put quote marks around it. The block quote and the colon that introduces it are enough to show that it is a quote.
Subtitles
Introduce a subtitle or subheading
Writing at Work: How to write clearly, effectively and professionally
Policy analysis: concepts and practice

It is common in the academic world in particular for titles to have a main heading and a subheading. In that case, separate them with a colon. Where you do so, conventional practice is to start the subtitle with a capital letter. The ASG's Referencing category discusses this further.

Modern Manglish: Gobbledygook made plain
Modern Manglish: gobbledygook made plain

Less commonly, professional documents will want to separate 2 parts of a heading. In this case, the second part does not need to start with a capital letter because it does not represent a formal published title.

Introduction: a new approach to old problems
Introduction: A new approach to old problems
In the days of the typewriter, colons were often used with capital letters to format a heading and separate it more prominently from the text. This is no longer necessary as word processors offer a wide range of formatting options, including font size and bold. Do not use a colon at the end of a heading.
Audit findings
AUDIT FINDINGS:
Ratios
Separate the numerals in a ratio, without spaces
You have a 50:50 chance of winning.
See also:   Dashes - – —,  Lists,  Publication titles,  Quotes,  Semicolons ;