The correct use of that and which is tricky. The word that you choose depends on the situation—in the U.S., at least. Both that and which are relative pronouns (as described here) that introduce a dependent clause.
A dependent clause is a part of a sentence that has a subject and a predicate but that cannot stand alone. For example, each sentence below contains a dependent clause (in bold):
The girl had a dragon tattoo that frightened her employer.
The girl wore a pearl earring, which the artist used as a focal point.
So what’s the difference? The clause in the first example is grammatically restrictive. Restrictive clauses are essential to the theme of sentence. In the example, dragon tattoo is given more meaning by the addition of that frightened her employer. The structure of the sentence gives more thematic weight to dragon tattoo than to other elements in the sentence.
In the second example, the clause is grammatically nonrestrictive. Nonrestrictive clauses are not essential to the sentence as a whole. The artist used the girl’s pearl earring as a focal point, but that is not the most important point of the sentence. The fact that the girl wore a pearl earring, however, is. The information in the which-clause is almost an afterthought.
Notice the comma before the which-clause (nonrestrictive). There is no comma before a that-clause (restrictive).
The Correct Use of that and which
So how do you decide whether you need that or which? Just ask yourself the following question: Am I presenting essential information? If so, use that. If not, use which.
Here’s another pair of example sentences:
The app that Chris downloaded yesterday doesn’t work.
The organization, which was founded in 1934, provides funding to needy families.
In the first example, the clause contains essential information: the app in question is the one that Chris downloaded yesterday. Presumably, all other apps that Chris has downloaded work.
In the second example, the clause contains nonessential information. We don’t need to know when the organization was founded to know what its mission is.
Note: If you’re British, the correct use of that and which is murkier. To my knowledge, British English doesn’t make a distinction between that and which. The following sentence, which was taken from a recent Economist article, uses which for a restrictive clause and that for a second (embedded) restrictive clause:
Local dealers then cut it with other chemicals such as benzocaine, a local anaesthetic which simulates the numbing effect that real cocaine has on the gums.
For more on writing effectively, see Why Good Copy Is Good Business: An Overview of Best Practices for Written Communications.
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