Items in a series
Here, the comma may be considered to stand for a missing ‘and’ or ‘or’.
- John mowed the lawn, Mary did the cooking and Frank lazed around.
- He came, saw and conquered.
- The committee considered sugar, beef and milk products.
An additional comma may be inserted before the final ‘and’ (or ‘or’) if needed for emphasis (see also below) or for clarification:
- sugar, beef and veal, 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
In the second example, ‘stable’ modifies ‘agricultural prices’, i.e. the phrase cannot be read as ‘stable and agricultural prices’.
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.
Note that if the subject of the second sentence is omitted, or if the conjunction is ‘and’ or ‘or’, 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.
In both cases, the considerations set out under bellow apply.
Parenthetic and introductory phrases
If a phrase is intended to complement or introduce the information in a sentence and has a separate emphasis of its own, it is set off by a comma, or by a pair of commas if inside the sentence:
- Mindful of the need to fudge the issue, the committee on commas never came to a conclusion.
- The committee on commas is composed of old fogeys, as you know.
- The committee on commas, however, was of a different opinion.
Note that the sentence must remain a complete sentence even if the parenthetic or introductory phrase is omitted.
Parenthetic phrases may also be created by setting off part of the sentence with a comma (or commas) while retaining the normal word order. Both the following are possible:
- The President was a great man despite his flaws.
- The President was a great man, despite his flaws.
Without the comma, the phrase ‘despite his flaws’ 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 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 2003, the committee took three decisions.
- In 2003 the committee took three decisions.
Parenthetic phrases (but not introductory phrases) may sometimes be marked by dashes.
Non-defining relative clauses
Non-defining relative clauses are special cases of parenthetic phrases. Note the difference compared with relative clauses that define the preceding noun phrase (i.e. ‘the translations’ or ‘the translations that have been revised’ in the examples below):
- The translations, which have been revised, can now be sent out. (added detail — they have all been revised)
- The translations which (or better: that) have been revised can now be sent out. (defining the subset that is to be sent out — only those that have been revised are to be sent out)
Note also that the use of ‘which’ in defining relative clauses is often considered to be stilted and overly formal. ‘That’ reads more naturally. It also helps make the meaning clearer, reinforcing the lack of commas, since it is used as a relative pronoun only in defining clauses. Unlike ‘which’, however, ‘that’ needs to be close to the noun to which it refers.
Combined uses of commas
The uses of commas described above can of course be combined. Worth noting is that an initial comma is not needed before introductory phrases in linked sentences:
- The committee dealing with the question of commas agreed on a final text, but despite the importance of the matter, the relationship with semicolons was not considered.
Avoid liberally sprinkling sentences with commas, but do so by constructing sentences so as to minimise the number of commas required rather than by breaching the comma rules described above. For example, inserted phrases can often be moved to the beginning of the sentence. Parenthetic phrases can also be rendered with brackets or dashes. Moreover, a parenthetic phrase may not in fact be appropriate. Finally, a complex sentence can be divided by a semicolon or even split into two or more sentences.