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Comma
Use a pair of commas to surround a non-defining clause (one which adds
descriptive information but which can be removed without losing the meaning
of the sentence) – note that only ‘which’ or ‘who’ can be used in this type of
clause, not ‘that’.
The library, which was built in the seventeenth century, needs to be
repaired.
The man, who climbed the tower without a safety harness, died of
old age.
Do not use commas to surround a defining clause (which cannot be removed
without losing the meaning of the sentence) – note that ‘which’ or ‘who’ can be
replaced by ‘that’ in this type of clause.
The library which was built in the seventeenth century needs to be
repaired [but the library which was built in the eighteenth century
does not].
The man that climbed the tower without a safety harness died of old age
[but the other man died in a different way].
He asked his friend Sam to be his second [not any of his other friends].
Use commas to surround a non-defining word or phrase (which adds
information but could be omitted without changing the sense of the sentence),
and follow the non-defining word/phrase with a single comma if it is at the
start of the sentence.
Shakespeare, the prolific playwright, might not have existed.
A prolific playwright, Shakespeare might not have existed.
He asked Sam, his friend, to be his second [not the Sam who is his
barber].
The prime minister, David Cameron, is an alumnus of Brasenose.
Do not use a comma where defining information is used at the start
of a sentence.
The prolific playwright Shakespeare might not have existed.
The prolific playwright, Shakespeare might not have existed.
His friend Sam was his second.
His friend, Sam was his second.
Defining vs non-defining information
Do not use a comma to join two main clauses, or those linked by adverbs
or adverbial phrases (eg ‘nevertheless’, ‘therefore’, ‘however’). This is sometimes
referred to as ‘comma splicing’. Either use a semicolon or add a coordinating
conjunction (eg ‘and’, ‘but’, ‘so’).
Shakespeare was popular, and his plays were all profitable.
Shakespeare was popular; his plays were all profitable.
Shakespeare was popular, his plays were all profitable.
Use a comma after an introductory adverb, adverbial phrase or subordinate
clause; or use a pair of commas surrounding it if it is in the middle of a sentence.
However, it was too late for that.
It was, however, too late for that.
With his possessions in a bundle, Dick Whittington walked to London.
Dick Whittington, with his possessions in a bundle, walked to London.
Do not use a comma after a time-based adverbial phrase.
After playing tennis all day she was tired.
Whenever she went to the cinema she ate popcorn.
In 2010 the most popular game among children was hopscotch.
Use a comma between multiple qualitative adjectives (those which can be used
in the comparative/superlative or modified with ‘very’, ‘quite’ etc).
He was a big, fat, sweaty man with soft, wet hands.