Following is a brief explanation of the choice between which and that, with remarks about the appropriate punctuation with which, that, and who clauses.

A question asked frequently is: "When should I use which, and when should I use that?" Let us first acknowledge that many people do not observe this distinction. However, this loose usage is hazardous because writers who do this may confuse the reader or unintentionally distort the meaning. Such writers are depending on the punctuation to do all the work. The which versus that dilemma has to do with restrictive versus nonrestrictive clauses. To understand the correct choice of which or that, one must be able to distinguish between restrictive and nonrestrictive clauses. Here's the difference:

Restrictive Clause: Defines, limits, identifies. It is not parenthetical. It makes an essential difference in the meaning of the sentence. It is not set off by commas.
Nonrestrictive Clause: Describes, gives incidental information. Is parenthetical. It is not essential to the meaning of the sentence (it’s sort of a “by the way” clause). It is set off by commas.

In absolutely correct usage, restrictive clauses are introduced by that (never which), and nonrestrictive clauses are introduced by which (never that). In practice, one will see many restrictive clauses (we know they are restrictive because they aren’t set off by commas) beginning with which. This is technically incorrect and can utterly confuse the meaning, but it happens. Here are some examples of correct usage:

Restrictive: “Companies that manufacture widgets are going bankrupt.” The meaning: All widget manufacturers are going bankrupt. The limiting that clause (with no commas) restricts the companies that are going bankrupt to those that make widgets.
Nonrestrictive: “The company, which manufactures widgets, is going bankrupt.” Here the main statement is, “The company is going bankrupt.” The which clause is incidental, parenthetical, nonessential information. It just adds: “by the way, they make widgets.” This is the type of construction that uses which (and commas).

Restrictive: “The dog that has three legs is called Ahab.” This identifies the three-legged dog as the one called Ahab OR identifies Ahab as the three-legged dog versus other dogs.
Nonrestrictive: “The dog, which has three legs, is called Ahab.” The essential information is that the dog is called Ahab; the incidental information is that Ahab is a three-legged dog.

Restrictive: “The barber cut my hair that is grey.” The implied (unlikely) meaning: The barber cut only my grey hair.
Nonrestrictive: “The barber cut my hair, which is grey.” The implied meaning: The barber cut all my hair, and my hair is grey.

The same principle applies to who clauses. With these clauses the punctuation is even more vital because the commas (or lack of them) are the only way to tell restrictive from nonrestrictive (whereas, if used correctly, which and that will distinguish the clauses discussed above). Consider: “Customers who shop by mail were interviewed.” This is restrictive; it specifies that only those customers who shop by mail were interviewed. The restrictive who clause limits the interviewees to customers who shop by mail. “Customers, who shop by mail, were interviewed.” This is nonrestrictive; it says that customers were interviewed and further implies that all of these customers shop by mail. Put another way, it says that we interviewed customers, and, by the way, these are people who shop by mail. In this example, only the commas (or lack of them) differentiate the two sentences. If this were a which/that situation and not a who situation, the first sentence would use that, and the second would use which.

Once you grasp the logic behind use of which and that, it's not hard to remember. In practice, most people usually use that correctly but frequently use which when they should be using that.

Rich Turner