From Guide to Grammar and Writing
Originally Posted by nyggus
The word which can be used to introduce both restrictive and nonrestrictive clauses, although many writers use it exclusively to introduce nonrestrictive clauses; ... From World Wide Words: WHICH VERSUS THAT
The clause “which has lost its handle” is certainly restrictive. If you take it out, you are left with “A suitcase is useless”, obviously a different meaning to that intended. So, according to Fowler’s rule, the which ought to be that.
To judge from correspondence, and from comments ... from time to time, there is confusion over which of these words to use when introducing clauses that modify nouns. This is not surprising, as there has been a shift in usage this century and older style books give different advice from newer ones.
Older style guides make two firm points about the difference between the two types of clause:
- Restrictive clauses are introduced by that and are not separated from the rest of the sentence by commas.
- Non-restrictive clauses are introduced by which and must be separated by commas from the rest of the sentence to indicate parenthesis.
The problem is that few people have followed these rules systematically, and you can find lots of examples where the relative pronoun which
is used to start a restrictive clause. The 1965 edition of Fowler’s Modern English Usage
If writers would agree to regard that as the defining relative pronoun, and which as the non-defining, there would be much gain both in lucidity and in ease. Some there are who follow this principle now; but it would be idle to pretend that it is the practice either of most or of the best writers.
This is even more true today than when he wrote it and most modern style guides say that either relative pronoun can be used with restrictive clauses. For example, I found this sentence quoted approvingly as an example under the equivalent section in “Oxford English”:
A suitcase which has lost its handle is useless.
Read more about the 'which hunt' here.