Is ‘that’ always restrictive and ‘which’ non-restrictive?

by Jakub Marian

Tip: See my list of the Most Common Mistakes in English. It will teach you how to avoid mis­takes with com­mas, pre­pos­i­tions, ir­reg­u­lar verbs, and much more.

There are two basic types of subordinate (dependent) clauses in English. A clause is called non-restrictive if it adds only parenthetical (that is, additional, non-essential) information to the sentence. For example,

Edgar Allan Poe, who wrote The Raven, was an American writer and poet.

The sentence tells us that “Edgar Allan Poe was an American writer and poet. Oh, by the way, he also wrote The Raven.” Non-restrictive clauses are always separated from the rest of the sentence by commas or parentheses.

A clause is called restrictive if it contains essential information; if it cannot be removed from the sentence without “damaging” its structure. For instance,

The poet who wrote The Raven was also a novelist.

Here, if the clause “who wrote The Raven” were omitted, we would have no idea who the poet was supposed to be. “The poet who wrote The Raven” acts as a single unit synonymous with “Edgar Allan Poe”.

Essential clauses provide context without which the whole sentence would fail to communicate its intended meaning, and, consequently, we (almost) never use commas or parentheses to separate them from the rest of the sentence.

‘That’ always introduces a restrictive clause

In modern English usage, “that” always introduces a restrictive clause (due to which it is almost never set off by commas):

correct The box that lies on the table is empty.
wrong Windows 10, that is mostly being distributed for free, is the newest operating system by Microsoft.

In the first sentence, the subordinate clause specifies which box we are talking about, so using “that” without commas was appropriate. In the latter example, “that” was inappropriately used to introduce a parenthetical (non-restrictive) clause.

Restrictive ‘which’ – a transatlantic divide

In British English, it is absolutely fine to use “which” in restrictive as well as non-restrictive clauses:

The box which lies on the table is empty. (correct in British English)
Windows 10, which is mostly being distributed for free, is the newest operating system by Microsoft. (correct in all major English varieties)

There is a widely accepted prescriptive rule in the US that states that “which” is to be used only in non-restrictive clauses (such as the second one above). The first sentence above is usually perceived as incorrect in formal American English.

There are several issues with this rule. First of all, it is a prescriptivist rule, and most Americans don’t strictly follow it in speech (many being unaware of its existence altogether). Secondly, when “which” is combined with a preposition, noun, or a pronoun, it cannot be replaced by “that”, even when it introduces a restrictive clause (this is considered to be an exception to the rule), as in

correct The principle in which he believes has been proved wrong.
wrong The principle in that he believes has been proved wrong.
correct She didn’t tell me which computer she had used.
wrong She didn’t tell me that computer she had used. (doesn’t make sense)
correct I don’t know which one I want.
wrong I don’t know that one I want. (doesn’t make sense)

So, should you follow the rule or not? If you write for an American audience and don’t follow the rule, you run the risk of sounding uneducated to some of your readers, so it is better to follow it. If you write for a British audience, you can ignore it completely, but if you do follow it, it will not make your written English look less natural.

By the way, if you haven’t read my guide on how to avoid the most common mistakes in English, make sure to check it out; it deals with similar topics.