To judge from correspondence, people are confused about which and that and, especially, which one to use when introducing clauses that modify nouns. This isn’t surprising, as there has been a shift in usage over the past century or so and older guides give different advice from newer ones.
The usage is intimately tied up with the distinction that grammarians make between two types of clause, which they call restrictive and non-restrictive. A restrictive clause is one that limits, or restricts, the scope of the noun it is referring to. Take these examples:
The house that is painted pink has just been sold.
The house, which is painted pink, has just been sold.
In the first one, the clause “that is painted pink” is a restrictive clause, because it limits the scope of the word “house”, indicating that the writer doesn’t mean any house, only the one that has been painted in that particular colour; if he takes that clause out, all that’s left is The house has just been sold: the reader no longer knows which house is being referred to and the sentence loses some crucial information. In the second example the clause is non-restrictive: the writer is giving additional information about a house he’s describing; the clause which is painted pink is here parenthetical — the writer is saying “by the way, the house is painted pink” as an additional bit of information that’s not essential to the meaning and could be taken out.
Here’s another example:
Another cause of stress is a traumatic event that is out of the ordinary and has a major impact on the person’s life.
The argument here is that the clause “that is out of the ordinary and has a major impact on the person’s life” modifies and constrains “event”. It’s not just any event but one specific type of event, to the extent that the whole block from “event” onwards forms one idea. The clause is restrictive.
Older grammar books make two firm points about the difference between the two types of clause:
This makes the whole matter seem neat and simple. But few writers have ever followed these rules systematically, and it’s easy to find examples in which which is used to start a restrictive clause. Sir Ernest Gowers, writing in the 1965 edition of Fowler’s Modern English Usage, comments rather sadly about this situation:
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; most modern grammar guides have caught up with the way people actually use the language and now say that either relative pronoun can be used with restrictive clauses. As an example, I found this sentence quoted approvingly under the equivalent section in Oxford English:
A suitcase which has lost its handle is useless.
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. According to the traditional rules, the which ought to be that. Note, however, that there’s no problem understanding what the writer means!
Despite the grammatical shift, there remain some situations in which that is still regarded as preferable to which, though they’re difficult to tie down. Here are some instances, but don’t take them as a full list of cases, and they are tendencies, not full-blown rules:
It seems likely this preference is partly derived from stress and rhythm. The word that contains “soft” sounds and is usually unstressed, whilst which has a “harder” initial sound and is easier to stress. Several writers note that that tends to be preferred in speech; this may be due to the comparative ease with which that is and similar phrases can be contracted, for example to that’s, compared with the equivalents using which.
One key proviso: though you can use which instead of that in restrictive clauses, you can’t do so the other way round: non-restrictive clauses ought always to start with which. Also, you can’t change the punctuation rules; it is particularly important to watch this point if you decide to use which in a restrictive clause, as otherwise your poor reader has no clue at all how you intend the sentence to be read. Here is a rather artificial example to make the point:
The cup which he stepped on is in the bin.
The cup, which he stepped on, is in the bin.
In the first, you’re being told about a specific cup with the special property that it is the one he stepped on; in the second, the fact that he stepped on it is an ancillary bit of information. My view is that punctuation is more important than choice of pronoun in such situations. You won’t be thought wrong if you use that in the first case (and will avoid the thunder of pedants’ condemnation) but you will be justly criticised if you leave out the commas in the second.
A further point worth noting is that the opening pronoun in restrictive clauses is frequently left out, so that you can say “The cup he stepped on is in the bin”. Again, you can’t do this with non-restrictive clauses.
If you wish to write naturally, don’t fuss too much about the usage of that versus which. Obsessive correction (sarcastically called a which hunt) is best avoided. If your sense of the language is not strong enough to be sure of the right pronoun, use that for the restrictive cases and which for the others and you won’t go wrong.