That Versus Which
The traditional approach to this question is to use "that" with restrictive clauses and "which" with nonrestrictive clauses. While some writers seem to have abandoned the distinction entirely, no better rule has come along to replace the traditional rule. Moreover, the rule is easy to master.1. Use "that" with restrictive clauses. A restrictive clause is one that limits -- or restricts --the identity of the subject in some way. When writing a restrictive clause, introduce it with the word "that" and no comma. (However, if the subject is or was a human being, use "who" to introduce the clause.)
Correct Restrictive Use:
The painting that was hanging in the foyer was stolen.
Explanation: The use of "that" in this
sentence is correct if the reader intends to single out the one painting
that was in the foyer as the stolen painting. However, if there were several
paintings hanging in the foyer, this use would be incorrect, since it would
mislead the reader into believing that there had been only one painting
in the foyer. The restriction here tells us that the one painting that
had been hanging in the foyer was stolen -- not the painting in the living
room, or the one in the drawing room, or any of those in the parlor.
2. Use "which" with nonrestrictive clauses. A nonrestrictive clause may tell us something interesting or incidental about a subject, but it does not define that subject. When writing a nonrestrictive clause, introduce it with "which" and insert commas around the clause. (However, if the subject is or was a human being, use "who" to introduce the clause and insert commas around the clause.)
Correct Nonrestrictive Use:
The painting, which was hanging in the foyer, was stolen.
Explanation: While this nonrestrictive
use tells us that the painting was hanging in the foyer, it does not tell
us which of the several paintings in the foyer was the stolen painting.
It would be incorrect to use this nonrestrictive clause if there had been
only one painting in the foyer, as the sentence leaves open the possibility
that there were others.
3. Combining Restrictive and Nonrestrictive Clauses. One can provide both limiting and nonlimiting information about a subject in a single sentence. Consider the following.
Correct Use of Both Restrictive and Nonrestrictive Clauses:
The Van Gogh that was hanging in the foyer, which we purchased in 1929 for $10,000, was stolen.Explanation: The restrictive clause beginning with "that" tells us that there was only one Van Gogh hanging in the foyer and that it was stolen. The nonrestrictive clause beginning with "which" tells us what the owner had paid for the painting, but it does not tell us that the owner did not pay another $10,000 for another painting in the same year. It does not limit the possibilities to the Van Gogh that was in the foyer.
4. Restrictive and Nonrestrictive Clauses beginning with "Who." When writing about human beings, we use "who" rather than "that" or "which" to introduce a clause telling us something about that human being. Since "who" is the only option, we distinguish between a restrictive use and a nonrestrictive use by the use of commas.
Correct Restrictive Use:
The suspect in the lineup who has red hair committed the crime.Note how the subject "suspect" in this sentence is restricted in two ways: we know that this suspect is both in the lineup and has red hair. As a result, we know that the other suspects, who are not in the lineup, could not have committed the crime. Moreover, of those suspects in the lineup, we know that the one suspect in the lineup with red hair committed the crime. If there were more than one suspect in the lineup with red hair, the above usage would be incorrect because it implies a different meaning.
Correct Nonrestrictive Use:
The suspect in the lineup, who owns a red car, committed the crime.In this example, the restrictive clause "in the lineup" tells us that of all possible suspects in the world, the one who committed the crime is in the lineup. However, while the nonrestrictive clause "who owns a red car" tells us something about the suspect, it does not foreclose the possibility that there are several different suspects in the lineup with red cars. The car color may tell us something useful, but it does not restrict us to only one possibility.
Cross Reference: Clauses -- Restrictive and Nonrestrictive
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