That, which, and who are relative pronouns that can introduce relative clauses.The traditional approach is to use "that" with restrictive clauses (see below) and "which" with nonrestrictive clauses. Some writers have abandoned the distinction entirely, but the traditional 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.
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.
relative pronouns, introducing descriptive clauses.
The issue involves what are called “restrictive” and “nonrestrictive” clauses. For clarity and consistency, our style rules call for a clear distinction in handling these constructions. Here’s what The Times’s stylebook says:
This rule is often neglected in speech and colloquial writing, and some usage manuals take a laissez-faire approach (see, for example, the American Heritage Book of English Usage, which offers a good explanation but no firm guidance).
But preserving the distinction adds polish and helps to clarify the desired emphasis in a sentence.
A couple of recent missteps:
The depths of G.M.’s problems came to light in its federal filing that painted a bleak picture of a company that has lost more than $20 billion this year and is in danger of not being able to pay its bills in a few weeks.
There’s only one federal filing involved, and it has already been described in the story. So the phrase “its federal filing” is fully specific; the relative clause adds additional information, but is not necessary to specify which filing we mean. So we should put a comma after “filing” and introduce the clause with “which.”
Starbucks reported flat revenue and a 98 percent decline in profit over the fourth quarter of 2007, which were dragged down in part by charges related to its turnaround plan that includes the termination of 1,000 employees.
Ditto. The turnaround plan has already been specified — in fact, the “its” alone serves this purpose. So what follows is a “nonrestrictive” clause adding more information; it should start with “which” and be set off with a comma. If we said “related to a turnaround plan,” then “that” would be appropriate.
The 2008 race for the White House that comes to an end on Tuesday fundamentally upended the way presidential campaigns are fought in this country, a legacy that has almost been lost with all the attention being paid to the battle between Senators John McCain and Barack Obama.
This may be a closer call. But “2008” specifies which race we’re referring to; there’s only one 2008 race for the White House. So in this construction, the next clause is nonrestrictive. It should be introduced by “which” and set off with commas.
Probably, though, we wanted to emphasize the “comes to an end” clause — this ran on Election Day — and make that the defining quality of “race.” In that case, we could have eliminated the previous limiting modifier “2008” and said, “The race for the White House that comes to an end on Tuesday …” (There have been many races for the White House; we’re talking specifically about the one that comes to an end Tuesday.)
The other relative clause—“that has almost been lost …”—is restrictive, and so is correct as it stands.
If you are defining something by distinguishing it from a larger class of which it is a member, use “that”: “I chose the lettuce that had the fewest wilted leaves.” When the general class is not being limited or defined in some way, then “which” is appropriate: “He made an iceberg Caesar salad, which didn’t taste quite right.” Note that “which” is normally preceded by a comma, but “that” is not.