EXPLANATION Adjectival clauses, which exist to provide information about nouns, are introduced by pronouns such as that, which, who, whom, whose. Those pronouns are called relative pronouns because they relate to (stand for) a noun in another clause, usually the principal clause.
This section looks at how to choose between who, that and which. Who is used when the relative pronoun is referring to a person. In the typical error above, the noun is the name of the person. Thus who should be used, not that. Another example is:
The relative pronouns that and which are generally used with nouns that do not involve people. However, there is a subtle difference in their usage, as is seen if we examine these two sentences.
- This is the house that Jack built. Our house, which was built forty years ago, is in need of renovation.
The commas give us a clue to the difference. In the first sentence, the clause that Jack built defines which house. It is not any house; it is the house that Jack built. When the adjectival clause is providing essential information and cannot be left out, it is called a defining clause. We generally use that to introduce defining clauses.
On the other hand, if the information being provided is non-essential (non-defining), we use which to introduce it and commas to separate it from the rest of the sentence. It is not possible to substitute that for which in the second sentence. We cannot say:
While it is possible to use which in the first sentence, many writers prefer to preserve the distinction between that and which to aid with clarity. Grammarians do differ on finer points of usage with the various relative pronouns. However, if you use who for people, that to introduce defining clauses and which to introduce non-defining clauses, you cannot go wrong. (See Exercise 8.5 for a discussion of the difference between who and whom.)