Do you sometimes wonder whether to use that or which in a sentence? The key to understanding proper usage of these words is learning the difference between restrictive and nonrestrictive relative clauses. Consider:
√ She held out the hand that was hurt.
In this sentence, that is introducing 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 result in either a sentence of doubtful meaning or a sentence that makes no sense at all:
X She held out the hand.
Restrictive relative clauses are typically introduced by that, as well as by whose, who, or whom. Note that in British English, the word which is often used interchangeably with the restrictive that:
√ She held out the hand which was hurt.
This common British construction is not strictly incorrect in American English, but it is generally avoided, especially in formal writing.
The other type of relative clause is a nonrestrictive relative clause. This kind of clause contains extra information that could be left out of the sentence without affecting the meaning or structure. Nonrestrictive relative 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.
If you leave out this type of clause, the result is a sentence that may have less information but still makes sense:
√ A list of contents would have made it easier to steer through the book.
√ She held out her hand.
Note that a nonrestrictive clause is preceded by a comma (setting off the extra information), whereas no comma should precede a restrictive clause (indicating that the information is essential, not extra):
They got into the van, which had Ohio plates. [nonrestrictive]
I was driving the van that had Ohio plates. [restrictive]
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