‘[commas] have two kinds of duty. One is to show the construction of sentences—the “grammatical” duty. The other is to introduce nuances into the meaning—the “rhetorical” duty.’
The Complete Plain Words, p. 155.

NOTE:   References to style manuals and grammar reference books given below don’t, in every case, signify the use of a direct quote from that manual.

Before joining words

Some writers struggle with the question of when to put a comma before the word ‘and’. Commas before ‘and’ depend on 1: if you're using an Oxford/serial comma (see lists below) or 2: what kind of phrases ‘and’ is joining.

According to The Little, Brown Handbook (Fowler and Aaron, 12th ed.), the rule is:

‘Two words, phrases, or subordinate clauses that are connected by a coordinating conjunction are not separated by a comma.’

E.g. ‘Sally was happy and excited.’
However,

‘When two main clauses are joined into one sentence with a coordinating conjunction, a comma precedes the conjunction.’

E.g. ‘Harry Potter was a publishing phenomenon, and he earned J.K. Rowling a lot of money.’

In lists

In a series of three or more items, no comma separates the final two items in the series.

e.g. apples, bananas and nectarines

In American English there usually is a comma before ‘and’ in a list.

Joining parts of a compound sentence (coordinate clauses)

E.g.    ‘The risks of funding the production of a book are considerable, and publishers generally suffer losses on a certain percentage of their publications.’

Note: there’s a comma before ‘and’ when joining two distinct parts of a sentence. Can also apply to the use of or, but, nor, neither, so, for and yet, as appropriate.

Longman, p. 148 (item marked 2) 
Style Manual, p. 76 (6.30)

Like ‘and’, ‘but’ is a conjunction—a word to join phrases into a sentence. A comma breaks sentences apart. Putting commas before ‘and’ and ‘but’ may be acceptable in American English, but it is not in Aussie English, although there are times you might have a comma before them.

Lin M. Hall, in correspondence

Where such initial clauses are short and are linked generally by ‘and’, the comma is frequently omitted.

E.g.   ‘The rain beat down and the wind howled.’

Style Manual, p. 77 (6.31)

The comma splice

Writers often use a comma when a full stop is required and this is known as a comma splice.

Incorrect: ‘The house is large, it has only three bedrooms.’
Correct: ‘The house is large; it has only three bedrooms.’
Incorrect: ‘A popular method of proofreading is by using spell checker, this method is ineffective at picking up all spelling and typing errors.’
Correct: ‘A popular method of proofreading is by using spell checker. This method is ineffective at picking up all spelling and typing errors.’
Incorrect: ‘The driver, Eddie Books, has never had an accident, nevertheless, accidents are always a risk.’
Correct: ‘The driver, Eddie Books, has never had an accident; nevertheless, accidents are always a risk.’

Longman, p. 148/149

Longman, p. 626/7

The Complete Plain Words, p. 156 (i)

With appositions

Use commas to separate off a non-defining apposition.

E.g. E.g. Editing For Beginning Writers, one of many books on editing, is available from the Amazon Bookstore.’
[just gives additional info. about the subject—commas needed]

Don’t use commas when the apposition is defining.

E.g. ‘Mark Stapler the electrician is not to be confused with Mark Stapler the TV presenter.’
[integral part of the sentence—no commas needed]

Choosing your Mark, p. 24 (4)

Parenthetical elements

Pairs of commas are used to set off parenthetical words, phrases and clauses.

E.g. ‘It seems to me, children, that you have not done your work.’ 
‘Exercise, no less than one’s diet, should be considered.’

Style Manualp. 80 (6.50)

The Complete Plain Words, p. 156 (ii)

Subject/verb; verb/object

Apart from such parenthetic uses, a comma should not come between the subject of a sentence and its verb, or a verb and its object.

E.g. ‘The people who are making the most money [no comma] pay the highest taxes.’

Longman. p. 149

Care should be taken not to flout this rule when a sentence is particularly long—it still applies.

E.g. ‘Pensioners whose aids require battery types not normally stocked will still have to buy their batteries from commercial outlets.’

Style Manual, p. 83 (6.71)

Compound predicate/compound sentence

Another error commonly made is to confuse a compound predicate with a compound sentence, and consequently to misplace the comma.

Correct: ‘We ran to the door and, finding that it was locked, began to search for another way of entry.’
Incorrect: ‘... door, and finding that it was locked, began ...’

In the example above, the ‘core’ sentence (without the addition of the clause ‘finding that it was locked’) would have read: ‘We ran to the door and began to search …’ This core sentence would not have taken a comma before the ‘and’, because not only is the initial part of the sentence short, but both ‘ran’ and ‘began’ qualify the same subject (‘we’). So when the addition of the clause (‘finding that …’) is made, there is still no need for a comma before the ‘and’, and the addition is made parenthetically—i.e. just the additional clause itself is surrounded by commas.

Correct: ‘We ran to her aid, but because the door was locked, precious minutes were lost.’
Incorrect: ‘... aid but, because the door was locked, precious …’

In the example above, the ‘core’ sentence (without the addition of the clause ‘because the door was locked’) would have read: ‘We ran to her aid, but precious minutes were lost.’ This original sentencewould have taken a comma before the ‘but’, because, while the verb ‘ran’ qualifies the subject ‘We’, the verb ‘were’ qualifies a totally new subject introduced in the second part of the sentence. So when the addition of the clause (‘because the door …’) is made, there is still the same need for a comma before the ‘but’.

Style Manual, p. 83 (6.72)

Introductory and transitional elements (of course; in fact; no doubt; therefore; meanwhile; however; otherwise; too; though; nevertheless)

A comma or commas need not always be used after such words and phrases as ‘therefore’, ‘meanwhile’ and ‘no doubt’.

E.g. ‘Therefore, it must always be borne in mind that things will not change unless we make them change.’
‘The article therefore has little interest for editors.’ 
‘The article is of little interest, therefore, to editors.’
‘He therefore decided to stay.’ [without comma, emphasis is on ‘therefore’]
‘He, therefore, decided to stay.’ [with comma, emphasis is on ‘He’ 
(as opposed to someone else)]
‘Meanwhile, if no action is taken, a crisis may occur.’
‘Meanwhile the situation is deteriorating.’
‘There are, no doubt, two sides to the question.’
‘No doubt there are two sides to the question.’
‘However you look at it, it’s illegal.’
‘However, if you really think about it, it’s illegal.’
‘Do you want to do that too?’ [without comma, emphasis is on ‘you’ 
(as opposed to someone else)]
‘Do you want to do that, too?’ [with comma, emphasis is on ‘that’]

Style Manual p. 79 (6.45) 

The Complete Plain Words, p. 162 (b)

A clause which includes a mobile conjunction must never be separated from the main part of the sentence simply by a comma. A weightier punctuation mark is necessary.

E.g. ‘They were late; however, they dawdled.’
‘They were late. However, they dawdled.’ [Both acceptable]
‘They were late; they therefore hurried.’
‘They were late. They hurried therefore.’ [Both acceptable]

Choosing your Mark, p. 27 (2)

Introductory and transitional elements (for example; for instance; thus; hence; namely)

Depending on context, these can often be used without a comma following. It is the complete element (including items in a series) that should be treated parenthetically:

E.g. ‘Some groups, for example self-publishing writers, would be adversely affected.’
But ‘Self-publishing writers, for example, would be adversely affected.’

Style Manual, p. 79 (6.46)

Or

When ‘or’ introduces an alternative, commas are not used.

E.g.   ‘Shall we go to the movies or take a walk?’

Style Manual, p. 80 (6.52)

Etc. (means ‘and other things’)

Do not use ‘etc.’ at the end of a list beginning with expressions such as ‘including’, ‘such as’ or ‘for example’.

Longman, p. 253 (item marked 3)

In a list, put a comma before ‘etc.’ unless only one item has been mentioned:

E.g. ‘Cats, dogs, birds, etc.’ 
But ‘Cats etc.’

Longman, p. 253 (item marked 3)

Commas in list of adjectives

When a number of adjectives precede a noun they qualify, all but the last adjective is separated off by a comma, except in instances where the mental substitution of ‘and’ would not make sense (i.e. depends on relationship to the noun).

E.g. ‘a low, gnarled bough.’ 
But ‘Professional financial teaching’ [commas here would suggest that the teaching is professional and also financial—which is true, but the relationship here is closer than that: it is financial teaching that is specifically professional—i.e. the adjective defines another adjective as much as it defines the noun.]

Style Manual, p. 81 (6.59)

Introductory adverbial phrases

Short introductory phrases don’t take a comma unless followed by a number, or could be (even momentarily) ambiguous.

E.g. ‘By 1980 there were 333 editors.’ 
‘By 1980, 333 editors had registered.’
‘In winter few continue to write outside.’
‘In winter, clothing contributions are even more gratefully received.’
‘In the 1950s many still enjoyed reading books of the war era.’
‘In the 1950s, war books were still popular.’

Style Manual, p. 79 (6.43)

Medium/long introductory adverbial phrases are usually marked off by a comma.

E.g.   ‘To prepare a paste up, follow this procedure.’

Style Manual, p. 79 (6.42)

An adverbial phrase that comes between the subject and the verb is usually marked off by commas.

E.g.   ‘Good students, after studying their standard texts, make use of the library.’

Style Manual, p. 79 (6.44)

Introductory adverbial clauses

An introductory adverbial clause is usually marked off from the main part of the sentence by a comma. If the clause is very short and no ambiguity could result, the comma may be omitted.

E.g. ‘Before you try to develop a completely new process, you should analyse reader requirements.’ 
‘When we wake up, the birds haven’t started singing.’

When introductory adverbial clauses express (continuing) time (e.g. using ‘as’, ‘since’, ‘while’), no comma is needed unless short or momentarily ambiguous.

E.g. ‘Since you came you have been an asset to us.’
‘While we were walking in the park the fireworks were exploding over the harbour.’ ‘While we were walking in the park, exploding fireworks were illuminating the north side of the harbour.’

Style Manual, p. 78 (6.38)

When an introductory clause expresses cause or condition, it is followed by a comma.

E.g.   ‘As it is so long since you visited us, I think you should stay overnight.’

Style Manual, p. 78 (6.40)

Addresses

Commas should not be used in addresses, unless they are written on the one line.

E.g. Mrs P Johnson 
30 Time Court 
Mareena QLD 4118
But: Mrs P Johnson, 30 Time Court, Mareena QLD 4118.

Choosing Your Mark, p. 21 (item marked 5)

Sentence adjuncts

It is common practice to use commas to separate off a sentence adjunct from the rest of the sentence.

E.g. ‘Yes, ‘I’ll edit it.’
‘First, we must edit the manual.’

However, Publish My Book.Online prefers to leave this optional, depending on context.

Choosing Your Mark, p. 24 (8)

Vocatives

Use commas to separate off a vocative.

E.g. ‘Jo, where are you going?’
‘Why were you, Jo, the one to tell me?’

Choosing Your Mark, p. 23 (3)

The absolute construction

Use commas to separate off the absolute construction.

E.g. ‘The proofreading work being done, we can all go home.’
‘We had to go to a freelance editor, the agency being closed.’

Choosing Your Mark, p. 25 (11)

Question tags

Use commas before a question tag (the two or three words tagged on to the end of a statement to turn it into a question).

E.g. ‘They are here, aren’t they?’
‘They aren’t late, are they?’

Choosing Your Mark, p. 25 (13)

Segments that share the same preposition

Use commas to separate off sentence segments that share the same preposition.

E.g.   ‘We travelled in the direction, and to the vicinity, of the bridge.’

Choosing Your Mark, p. 25 (14)

Segments that share the same prepositional object

Use commas to separate off sentence segments that share the same prepositional object.

E.g.   ‘She flew over, and he sailed under, the Wordsmith Bridge.’

Choosing Your Mark, p. 25 (15) 
Style Manual p. 82 (6.60)

Ambiguity

Use commas to avoid ambiguity.

E.g. ‘Joe, said Iris, was very helpful.’ 
‘Joe said Iris was very helpful.’

Choosing Your Mark, p. 25 (18)

‘He was not run over, mercifully.’ 
‘He was not run over mercifully.’

Style Manual, p. 83 (6.67)

Whereas it is standard practice to omit the comma before the ‘and’ ending a list of adjectives, should there be any potential ambiguity (i.e. the final element or the one preceding it has itself got ‘and’ in it), a comma should be used.

E.g. ‘There were many expeditions, including those of Sturt, Mitchell, Burke and Wills, and Darling,’

Style Manual, p. 81 (6.54) 
The Complete Plain Words, p. 164 (iv a)

Relative clauses/relative pronouns

A clause is called ‘relative’ when it relates to a preceding noun. Relative clauses can relate to nouns in one of two ways: as defining or as non-defining relative clauses. With the former, it is acceptable to use either ‘that’ or ‘which’ at the commencement of the clause (although some grammarians have a definite preference for ‘that’); with the latter, one must always use ‘which’. Note: ‘who/whom/whose’ is normally used in place of ‘that’ or ‘which’ when referring to a person.

Examples of defining/restrictive clauses (no comma needed):

[integral to the meaning of the sentence]

‘I want a book that/which is entertaining.’ 
‘I admire parents who look after their children well.’ 
‘Salaries that/which are paid monthly are augmented by fringe benefits.’

Examples of non-defining/non-restrictive/commenting clauses (comma always needed):

[clause just gives additional info about the subject]

‘You can have my book, which is pretty funny.’ 
‘I’ve just spoken to my parents, who have returned from overseas.’ 
‘You can take it out of my salary, which is paid monthly.’

Choosing Your Mark, pp. 16 (13)–18, 26 (22)
The Complete Plain Words, pp. 121–123, 158(iii)–159 
Style Manual, p. 77 (6.35–37)
Fowlerpp. 774 (3)–775 (4a)
Longman, pp. 605 (1) & 705 (3)

To separate off a quotation

Use commas to separate off a quotation after such words as ‘says’, ‘said’.

E.g.    ‘Jacqui said, “I’m glad it’s all over with.”’

Choosing your Mark p. 27 (23)

Note, use a colon (instead of comma) if making the quote a separate paragraph.

List elements that contain internal commas

Semi-colons should be used to separate sentence parts that contain internal commas.

E.g.    ‘Paris, TexasHello, Dolly; and The Good, the Bad and the Ugly.’

Choosing your Mark, p. 28 (4)

Clauses containing contrasting ideas

To separate clauses containing contrasting ideas, when there is no conjunction between the clauses, something weightier than a comma needs to be used.

E.g. ‘To err is human; to forgive is divine.’
‘To err is human. To forgive is divine.’ [Both acceptable]
‘I’ll take the high road; you’ll take the low road.’
‘I’ll take the high road. You’ll take the low road.’ [Both acceptable]

My preference, when the sentence elements are short, is for the semi-colon.

Compound verb structures

Only separate compound finite verb structures when another grammatical reason (e.g. parenthetic phrases/clauses) deems it necessary.

E.g.   Note the progression below:

‘He came to be our editor.’
‘He came to the office to be our editor.’ 
‘He came to this very office to be our editor.’ 
‘He came to this very office, though not many know it, to be our editor.’

Inverted names

Use a comma between inverted names.

E.g. Greenslade, Amanda 
Jones, Michael

Choosing Your Mark, p. 24 (5)

Repeated words

Use a comma between repeated words.

E.g. ‘They were very, very sad.’
‘Everywhere one looked, there were errors, errors and more errors.’

Choosing Your Mark, p. 24 (6)

Antithetical elements

An antithetical phrase or clause is marked off by a comma or commas, unless the phrases are very short and of parallel construction.

E.g. ‘We were supposed to be there for instruction, not for the entertainment of the so-called facilitator.’
‘Not more equipment but better trained workers was what the study recommended.’
‘We saw dyelines, not galleys.’
‘We saw not galleys but dyelines.’

Style Manual p. 82 (6.62)

Omissions

Commas are used to indicate the omission of one or more words common to two parts of a sentence, particularly where numerical expressions are involved.

E.g. ‘In 1983 there were fourteen publications; in 1984, twenty-seven; and in 1985, ten.’
‘In 1983 there were 90 publications; in 1984, 126; and in 1985, 144.’

Style Manual, p. 82 (6.65/60)

Numbers

Commas should be used to separate numbers that might otherwise cause momentary confusion.

E.g.   ‘The board said that in 1986, 200 more books would be produced.’

However, it is preferable to reword the sentence.

E.g.   ‘The board said that 200 more books would be produced in 1986.’

Titles

Commas should be used to separate names from titles or degrees that come after the name.

E.g. Captain Hornblower, RN
The Honourable Joseph Macquarie, MP

Books etc.

In Amanda Greenslade’s fantasy book, Talon … 
[non-defining, since she’s only written one book—therefore use commas]

In Maggie Furey’s book Aurian ... 
[defining, since she’s written more than one book—therefore no need for commas]

In the book 21 Ways to get Published, by Jake Smith, …
[no need for commas]

In the book 21 Ways to get Published…, by Jake Smith, …
[comma after the book title, but not before]

Either ... or; from ... to; whether ... or; both ... and

There should generally be no comma between these constructions (no matter how long the sentence is). Exception: where perhaps a parenthetic phrase comes in between them.

Broken quotes

[see Style Guide—Quotations section]

Logically connected statements

A comma should be placed between logically connected statements—i.e. phrases etc. that follow on logically from each other.

E.g.   ‘The higher the word-count, the higher the cost to edit it.’

With quoted terms

This relates to the placement of commas etc. when using quote marks to set off terms and slang words (e.g. ‘procrastilaxing’).

British/Australian and American grammar differs on this so we would normally use the British/Australian method.

British/Australian rule: Any punctuation mark that is required, by the context, to follow the term will be placed outside the quote marks.

E.g.   You may not have heard of the word ‘procrastilaxing’, but my husband invented it on Facebook.

    American rule:
Any punctuation mark that is required, by the context, to follow the term will be placed inside the quote marks.

E.g.   You may not have heard of the word ‘procrastilaxing,’ but my husband invented it on Facebook.

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