‘That’ or ‘which’?
✓ She held out the hand which was hurt.
✓ She held out the hand that was hurt.
In these sentences, that and which are introducing what’s known as a restrictive relative clause. This is a clause containing essential information about the noun that comes before it. If you leave out this type of clause, the meaning of the sentence is affected – indeed, it will probably not make much sense at all. Restrictive relative clauses can be introduced by that, which, whose, who, or whom.
The other type of relative clause is known as a non-restrictive relative clause. This kind of clause contains extra information that could be left out of the sentence without affecting the meaning or structure. Non-restrictive clauses can be introduced by which, whose, who, or whom, but you should never use that to introduce them. For example:
A list of contents would have made it easier to steer through the book, which also lacks a map.
She held out her hand, which Rob shook.
Note that a non-restrictive clause is preceded by a comma (so as to set off the extra information), whereas no comma should precede a restrictive clause (indicating that the information is essential, not extra):
I bought a new dress, which I will be wearing to Jo's party. [non-restrictive]
I was wearing the dress that I bought to wear to Jo's party. [restrictive]
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