Punctuation

Full stop
Colon
Semicolon
Comma
Apostrophe

The purpose of punctuation is to break up groups of words that would be confusing or difficult to read if left to run together. Accurate punctuation produces writing that is clear and fluent.

Punctuation marks act as signals to the reader; for example, a full stop signals the end of a sentence; a comma signals a break in connection between words or groups of words within a sentence; and a colon signals that an explanation or elaboration is to follow.

General advice on how to use some of some common punctuation marks is given below. Please note that this advice covers broad concepts and does not aim to be comprehensive.

Full stop

A full stop (.) signals the end of a sentence.

Use a full stop at the end of a complete sentence.
Do not use a full stop at the end of headings or table and figure titles that stand alone.
Do not use a full stop at the end of bulleted items unless they are complete sentences.

Colon

A colon is used to introduce a word, a phrase, a sentence, a quotation, or a list. What follows the colon should explain, define or elaborate on the word or words that precede the colon.

Semicolon

A semicolon (;) signals a break that is stronger than a comma, but not as abrupt as a full stop.

Use a semicolon to signal a break between two groups of words closely connected in thought that could otherwise stand alone as sentences.
His spirit was broken; he would never be able to return.

Not all of the experiments were completed; however, she had gathered enough evidence to reach a conclusion.
Use a semicolon to separate items within a list when the items themselves contain commas or other punctuation, or if they are lengthy.
The study covered visitors to the following locations: Ballarat, Victoria; Geraldton, Western Australia; and Eyre, Queensland.

Comma

A comma (,) separates individual words, or groups of words, within a sentence. Commas prevent sentences from being misread or misunderstood by signalling which words belong together, and which words do not. Commas are particularly important in long and/or complex sentences.

Use a comma to separate three or more items in a list.
The study focused on the economic impact of drought in Victoria, Queensland, South Australia, Tasmania and New South Wales.
Note: If the items themselves are long or already contain commas, use a semicolon to separate.
Use a comma to separate groups of words that could stand alone as sentences but are joined by conjunctions ('and', 'but', 'for', 'or', 'nor', 'yet', 'so'). In such cases, the comma before the conjunction tells the reader that the two parts are independent.
Recent research suggests that this analysis is expensive, and it is predicted that any conclusions will have limited application.
Use a comma to separate 'introductory' statements from the main part of a sentence. Introductory statements are incomplete and often begin with words such as 'because', during', 'while', 'after', 'although', 'as', 'if', 'since', and 'when'.
Because of the transport strike, the interviews were rescheduled.

If work is submitted late, students will be penalised.
Note: If the order of these two parts is reversed, a comma is not needed.
The interviews were rescheduled because of the transport strike.

Students will be penalised if work is submitted late.
Use a comma to separate words that add non-essential information or 'interrupt' the main part of a sentence. To test if words are non-essential, remove them and see whether the main meaning of the sentence changes.
The author, who is a fascinating man, gave a lecture on Thursday.

It was likely, however, that such a meeting would never take place.

The results, as you have probably noticed, need to be rechecked.

Students must avoid distractions, which can be a test of self-discipline.
Use a comma to separate contrasting groups of words.
The results were due to our work, not theirs.

Apostrophe

An apostrophe is inserted into a word to signal one of two things: missing letters (omission), or ownership (possession).

Use an apostrophe to indicate missing letters when two words are joined ('contractions'). Note that contractions are inappropriate in formal writing.
They're running late. [they are]

Who's coming to the lecture? [who is]

You're not welcome. [you are]

It's been a busy semester. [It has]

It's not over yet. [It is]
Use an apostrophe followed by an 's' to indicate ownership by someone or something. If the person or thing showing possession is plural and already ends in an 's', just add an apostrophe (without another 's').
The student's textbook was stolen. [the textbook of the student]

She attended the students' party. [the party of the students]

The nation's grief is palpable. [the grief of the nation]

The children's teacher has retired. [the teacher of the children]

The historians' dates were wrong. [the dates of the historians]
Do not use an apostrophe with a plural word that has nothing to do with possession.
The brochures displayed images of pizzas being served at outdoor cafés. [Not: brochure's, pizza's, café's]
Do not use an apostrophe with possessive pronouns (the words 'its' 'theirs', 'yours', 'ours', 'hers', 'his'). These words already indicate possession and do not require an apostrophe.
The printer needed its cartridge replaced. [Not: it's]

The mistake is yours. [Not: your's]
Note: If you are unsure whether to use 'it's' or 'its', apply this quick test: replace the 'its' or 'it's' with 'it is' or 'it has'. If the sentence makes sense, use 'it's'; if not, use 'its'.
Do not use an apostrophe with the plurals of capital letters and numbers.
We borrowed a number of DVDs from the library. [Not: DVD’s]

This area underwent a change in the 1980s. [Not: 1980’s]