Do you sometimes wonder whether to use that or which in a sentence? In many cases, in British English, both words are equally correct.
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 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 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 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]