Two recent visitors have asked what the difference is between which and that and when to use each.
People usually struggle with the distinction when they are used as relative pronouns beginning a relative clause, which acts as an adjective in a sentence.
Restrictive and Nonrestrictive Clauses
The pronoun which begins a nonrestrictive relative clause, which means that the clause only comments on or adds information about a noun. In other words, the clause is not essential; it’s not necessary to identify the noun that it is modifying. It’s descriptive only.
The pronoun that begins a restrictive relative clause. This type of clause restricts the meaning of the noun. It is essential to identify the noun that it modifies.
Punctuating Which and That
Because a clause beginning with which is not essential to the meaning or identification of the noun that it modifies, it is set off by commas. I used to tell my students to think of the commas as hooks that we put on either side of an “optional” group of words. The hooks suggest to us that we can pull the group of words out of the sentence and still retain meaning.
A restrictive clause is not set off by commas because it’s necessary for the meaning of the sentence. Think of it as a permanent fixture, one that is seamlessly integrated with the rest of the sentence.
Geoffrey K. Pullum, who is far more knowledgeable about such matters than I ever was, argues quite convincingly in his recent blog post, “Check all boxes,” that “it is the commas around non-restrictive relative clauses that identify them in written English, not whether the first word is which or that.”
And there is other evidence that supports the idea of matter composed of multiple neutrons: neutron stars. These bodies, which contain an enormous number of bound neutrons, suggest that as yet unexplained forces come into play when neutrons gather en masse.—Michael Brooks, 13 things that do not make sense [Brooks uses “which” with commas because “the bodies” have already been identified; he’s simply adding information about them.]
And when the parents wanted to take up the fight against the teacher’s lawsuit, they created their own Web site, which became a base for framing their argument in the news media.—Peter Boyer, Q. & A. – What Would Jesus Teach? [Here the “which” clause has an opening comma, but because it is located at the end of the sentence, it doesn’t have a trailing comma.]
After checking with a postal clerk about the legality of stepping up his efforts, he began cutting up magazines, heavy bond paper, and small strips of sheet metal and stuffing them into the business reply envelopes that came with the junk packages.—Ian Urbina, No Need to Stew [The relative clause here is necessary to identify the particular business reply envelopes.]
To coexist with loud cellphone talkers, the Web offers hand-held jammers that, although illegal in the United States, can block all signals within a 45-foot radius.—Ian Urbina, No Need to Stew [Part of this sentence is set off by commas, but that part is within the relative clause, not the clause itself. The relative clause begins with “that” and has no commas because it is identifying the particular hand-held jammers that the Web offers.]