Everything English

“That” is a restrictive, or defining, pronoun, meaning (basically) that it is used in definitional contexts:

• This is the hat that I wore to the wedding.

Here, “that I wore to the wedding” is a restrictive clause introduced by the restrictive pronoun “that”. The restrictive clause defines the hat as the one worn to the wedding.

(On a side note, you might also see: “This is the hat I wore to the wedding.”)

Whether “which” may also be used as a restrictive, or defining, pronoun is debatable. The previous example could be written as:

• This is the hat which I wore to the wedding.

However, this construction is used mostly in informal speech. Speakers of formal American English tend to consider “which” a non-restrictive, or non-defining, pronoun to be used in non-restrictive clauses that are set off by commas (in writing) or pauses in speech, as in:

• This hat, which I wore the wedding, will never be worn again.

The confusion, for some, arises in cases such as the following:

• The car, which is faster than all the others, will win the race.
• The car which is faster than all the others will win the race.

In the first example, a specific car is mentioned. It is implied that this car is faster than all the others and is predicted to win the race. In the second example, a general rule is stated – that the fastest car (not this car, but whichever car is fastest) will win the race. The meanings of the sentences are different. Generally, formal American English requires that the second example be written as:

• The car that is faster than all the others will win the race.

One way to think about the difference between restrictive and non-restrictive clauses – and when to use “which” vs. “that” – is to remove the clause in question from the sentence and see whether the meaning of the sentence changes:

• 1) The car, which is faster than all the others, will win the race BECOMES 2) The car will win the race.

The meaning doesn’t really change, as we began (1) by stating that “the car” (this car), will win the race – and added, with the sub-clause “which is faster than all the others” that the car happened to be faster than other cars. The fact that the car was faster was not essential to the sentence, whose purpose was to note that the car would win. (In this sense, noting that the car was faster was kind of a “by the way …”)

• 1) The car that/which is faster than all the others will win the race BECOMES 2) The car will win the race.

In this case, we began (1) by stating a rule: The fastest car will win the race. The clause “that/which is faster than all the others” was essential to the definition of this rule. By removing it, we changed the meaning of the sentence. (2) does not state that the fastest car will win. It says that a specific car (this car) will win.

Generally speaking, the rule is that when a removing a clause from a sentence alters the meaning of that sentence, that clause should off-set by “that”, not “which”. When the meaning of a sentence is unaltered by the removal of a clause, the clause should be off-set by “which”.