Dashes, semicolons, colons and three dots to extend sentences
 
Use a colon (:) to give more information about something mentioned previously in the sentence.
For example: He blamed his divorce on one thing: beer.

Use a semicolon (;) when a slight break is preferable to starting a new sentence. (A semicolon gives a smoother transition than a full stop / period.)
For example: The pilot was worried; the elevators were packed with ice.

Use three dots (...) as a pause for effect.
For example: Crime does not pay ... as well as politics.

If you can't remember any of the rules above, use a dash ( — ). It covers all three.
 

Using Colons

A sentence can be extended with a colon when the writer wishes to expand on something already mentioned in the sentence. (A colon is used to introduce some more information about something previously mentioned in the sentence.)

Examples:

There were two pets in the house: a budgie and a cat.
(two pets: a budgie and a cat)

He blamed his divorce on one thing: beer.
(one thing: beer)

Using Semicolons

A sentence can be extended with a semicolon when a slight break is preferable to a new sentence.

Examples:

No one was hurt in the accident; the only real injury was a broken finger.  

She did not approve of my design at all; she found it crass.  

Quite often, there is a short phrase immediately after the semicolon which acts like a bridge between both halves of the sentence. It is called a transitional phrase. (This is covered more in the lesson Semicolons before transitional phrases.)

Examples (with the transitional phrases shaded):

Everybody knows he is guilty; however, it will never be proven.

Her own guest was declined; as a result, she left.

Business is booming; for example, Siemens have rung twice
since 4 o'clock.  

This business will collapse if you do not invest in the staff's well-being; of course, that is just my opinion. 

Using Three Dots

If you want to create pause for effect, use three dots.

Examples:

I don't want to achieve immortality through my work…I want to achieve it through not dying. (Woody Allen)

She had a bath once a year…whether she needed it or not. (Mark Twain)

Bart, with $10,000, we'd be millionaires! We could buy all kinds of useful things like...love! (Homer Simpson)

As I moved the bushes, I was not confronted by the deer I had been tracking 
for two days but...a Bengal tiger. 


three dots used as a pause for effect (correct usage)
(newspaper article)

Using Dashes

Confused about colons, semicolons and three dots? Use a dash.  The dash performs all the functions of the colon, the semicolon and three dots mentioned above.

Examples:

He blamed his divorce on one thing – beer.  
(replaces a colon)

The sales team meet on Tuesdays – unlike the floor managers who meet on
Wednesdays.
(replaces a semicolon)

The team requires another person – namely, Adam Richards.
(replaces a semicolon)

No one was hurt – the only injury was a broken finger.

As I moved the bushes, I was not confronted by the deer I had been tracking for
two days, but – a Bengal tiger. (replaces three dots)

I've been looking for a girl like you – not you, but a girl like you. (Groucho Marx) 

I'm really a timid person – I was beaten up by Quakers. (Woody Allen)

A credit card stolen from a woman in Devon was used to pay for a Chinese 
meal 18 hours later – in Hong Kong.


Author has used a dash as a pause for effect.
Author could also have used three dots.
(magazine article)
 
A COLON IS LIKE AN EQUALS SIGN 

Many people like to think of a colon as an equals sign.  Look at the examples to the left.

two pets = a budgie and a cat
one thing = beer

DIFFERENT FROM SEMICOLONS 

A colon should only be used to extend a sentence when you are introducing words which expand on something previously mentioned. When a slight break is preferable to a new sentence, you should use a semicolon.

The pilot was worried: the elevators were packed with ice.  
 
 
IN APPOSITION 

The words after the colon are known as an appositive phrase. (It just means an equal phrase.) You can also use the term in apposition to. For example:

There was only one fish in the vicinity: a great white shark. 
(In this sentence, great white shark is in apposition to fish.)

This company has always had the same motto: Try it twice and then sack it. 
(In this example, Try it twice and then sack it is in apposition to motto.)

ANOTHER USE FOR 3 DOTS (CALLED ELLIPSIS) 

Three dots can also be used to show that words have been omitted. This is covered more in the lesson Quotation Marks.

The magazine claims: "The scene in the '70s was...controlled by The Ramones."
 
 
DON'T RELY SOLELY ON DASHES 

Although a dash covers the functions of a colon, a semicolon and 3 dots, it is worthwhile learning how all are used so you can choose the one that looks best in your sentence. In the example below, there are already several hyphens and dashes; so, using the 3 dots is preferable to another dash.

Mrs Thomas – the 64-year-old lady from Kent who swore she would never have a facelift – removed the bandages to
find...a 40-year-old version of herself.  
 

See also:

How to use colons to extend sentences
Run-on error with a comma
Commas after a transitional phrase
Conjunctions and commas
Conjunctions and semicolons
Using semicolons to extend a sentence