Although I have already exemplified the importance of commas with relative clauses, I did so jokingly. This post is a more sober take on the subject, which causes much confusion among writers. Relative clauses, which post-modify nouns and do the job of an adjective, begin with relative pronouns which, that, who, whom and whose. Such post-modification can be restrictive (aka defining) or non-restrictive (aka non-defining). Both types are exemplified below.
Restrictive (defining) post-modification by relative clauses (italicised)
The girl who used to live next door has moved away.
This is the photograph that we took in Spain.
Snakes which are poisonous should be avoided.
The chap whose ladder I have borrowed must be out.
The couple whom we met last week are coming to tea.
Restrictive relative clauses are essential to the meaning of the noun (or nouns) they modify. This is why they are NEVER set off by a comma – or commas.
Non-restrictive (non-defining) post-modification by relative clauses (italicised)
Susan, who used to live next door, has moved away.
Rattlesnakes, which are poisonous, should be avoided.
Mr Jones, who is our GP, is retiring soon.
Tom, whose ladder I have borrowed, is obviously out.
The Browns, whom we met last week, are coming to tea.
Unlike restrictive relative clauses, non-restrictive ones are always set off by commas. Why? Because we don’t need them to understand the rest of the sentence. In other words, whatever you can cut out of a sentence without changing its meaning you should separate by commas. Whether a clause is restrictive or non-restrictive has thus a direct bearing on sentence punctuation. This principle, fundamental though it is, is widely misunderstood, and blunders abound – some with seriously misleading, or unintentionally hilarious, consequences. Do you think any parent in their right mind would send their darling child to a school making this statement in its prospectus?
We will punish all children, who are disruptive.
No, me neither. But remove the comma, and the school might find itself oversubscribed.
We will punish all children who are disruptive.
And do you think our poorer senior citizens would vote for a party putting forward this proposal?
Pensioners, who don’t need free bus passes, should be deprived of this perk.
But they may well back a party if its manifesto declared this:
Pensioners who don’t need free bus passes should be deprived of this perk.
Punctuation blunders with relative clauses crop up all over the place. This is what I found in The Times Educational Supplement.
“Children are owed a duty of care by schools which are ‘in loco parentis’.”
Are they really suggesting that some schools are not responsible for children in their care? No, of course not; thus:
Children are owed a duty of care by schools, which are ‘in loco parentis’.
More examples can be found in Maxi-rant 12: Another elusive comma, published on 15 October 2014. Finally, I have a handy tip for you: whenever a relative clause post-modifies a name (which will, of course, be capitalised), you will need to put a comma after it because names are usually self-defining: whatever modification follows will merely supply additional details. This, again, is an area where writers often blunder. The sentence below comes from The Sunday Times.
“I like Theresa May who is tough.”
But what about all the other Theresa Mays? They may also be likeable. But, of course, this venerable newspaper did not mean what it said. Thus:
I like Theresa May, who is tough.
My final two examples come courtesy of The Economist.