Items in a series. In a list of two items, these are separated by ‘and’ or ‘or’:

The committee identified two errors in the document: the date of implementation and the regulation number.

In a list of three or more items, a comma is used to separate them, except for the final two which are separated by ‘and’ or ‘or’:

Robin mowed the lawn, Sam did the cooking and Kim lazed around.

The committee considered sugar, beef and milk products.

An additional comma (known as the Oxford comma or serial comma) can be used before the final item to help clarify the sense:

… sugar, beef and other meats, and milk products.

A comma also comes before ‘etc.’ in a series:

… sugar, beef, milk products, etc.

but not if no series is involved:

They discussed milk products etc., then moved on to sugar.

Commas also divide adjectives in series:

moderate, stable prices

but not if the adjectives do not form a series:

stable agricultural prices

Linked sentences. Use a comma to separate two sentences linked by a conjunction such as ‘but’, ‘yet’, ‘while’ or ‘so’ to form a single sentence:

The committee dealing with the question of commas agreed on a final text, but the issue of semicolons was not considered.

Where there is no conjunction, use a semicolon (see Section 10.1.5):

The committee dealing with the question of commas agreed on a final text; the issue of semicolons was not considered.

Note that if the subject of the second sentence is omitted, or if the conjunction is ‘and’, ‘or’ or ‘but’, the comma is not obligatory:

The committee dealing with the question of commas agreed on a final text[,] but did not consider the issue of semicolons.

The committee dealing with the question of commas agreed on a final text[,] and the Council approved it.

Parenthetic and introductory phrases. Parenthetic phrases may be created by setting off part of the sentence with a comma (or commas) while retaining the normal word order. The following are possible:

The committee adopted the proposal despite the hesitation of some members.

The committee adopted the proposal, despite the hesitation of some members.

Without the comma, the phrase ‘despite the hesitation of some members’ forms part of the statement. With the comma, the phrase complements it, i.e. the sentence retains its sense if the phrase is omitted. The comma is therefore correctly left out in the following sentence:

Phrases must not be set off by commas if this changes the intended meaning of the sentence.

However, a comma is required if the phrase has a separate emphasis simply by virtue of being moved out of position, for example to the beginning of the sentence:

If this changes the intended meaning of the sentence, phrases must not be set off by commas.

Note, though, that short introductory phrases (typically two to three words) need not have any separate emphasis of their own, i.e. they may be run into the rest of the sentence. Both the following are possible:

In 2012, the committee took three decisions.

In 2012 the committee took three decisions.

Parenthetic phrases (but not introductory phrases) may sometimes be marked by round brackets (see Section 10.1.7(a)) or dashes (see Section 10.1.8(a)).

Commas in relative clauses. Commas are used to make an important distinction between two types of relative construction, often known as ‘defining’ and ‘non-defining’ relative clauses. Compare the following sentences:

The auditors were not able to identify the cows which were on the mountain pasture at the time of the audit.

The auditors were not able to identify the cows, which were on the mountain pasture at the time of the audit.

In the first case – without a comma – the relative clause (which were on the mountain …) tells us which cows we are talking about. Probably, the auditors had been able to identify other cows (the ones at the farm); it was just the ones on the mountain that had escaped identification. This is called a defining relative clause.

In the second case – with a comma – the relative clause is non-defining. It adds extra information about the cows, but does not identify which ones are being talked about. In this case, the implication is that the auditors had not been able to identify any cows at all, because they were all on the mountain.

Where the presence or absence of a comma changes the meaning of the text, there is a risk that it will be misunderstood. Consider rewriting the text to remove all possible doubt:

The auditors were only able to identify the cows which were at the farm at the time of the audit; they were unable to identify those which were on the mountain pasture.

The auditors were not able to identify the cows, because they were on the mountain pasture at the time of the audit.

Non-defining relative clauses are also used parenthetically. Compare the following:

My uncle, who lives in America, is rich.

My uncle who lives in America is rich.

In the first case, I only have one uncle (he lives in America, by the way) and he is rich. In the second case, I have a number of uncles, but the one that lives in America is rich; we know which uncle is being referred to because he is defined by the relative clause.

Note that the relative pronoun ‘that’ can be used (instead of ‘which’ or ‘who’) in defining relative clauses, but not in non-defining relative clauses. Therefore we can have:

The auditors were not able to identify the cows that were on the mountain pasture at the time of the audit

but not:

The auditors were not able to identify the cows, that were on the mountain pasture at the time of the audit.