Comma before ‘that’ and ‘which’

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 (PDF Version).

What distinguishes English from most other languages is its use of commas before a dependent (subordinate) clause. Dependent clauses (clauses introduced by words like “that”, “which”, “who”, “where”, “how”, etc.) are usually neither preceded nor followed by a comma. For example:

Cars that don’t have seat belts aren’t allowed to carry children. (correct)
Cars, that don’t have seat belts, aren’t allowed to carry children. (wrong)
I don’t know which one I want. (correct)
I don’t know, which one I want. (wrong)
Could you tell me where it is? (correct)
Could you tell me, where it is? (wrong)

Dependent clauses are (and in fact must be) separated with commas only when the information contained in the clause is not important for the overall meaning of the whole sentence. A good way to recognize such clauses is to try to enclose the clause in parentheses; if the sentence still makes sense, you should use commas (or parentheses) to separate the clause from the rest, e.g.

Brazil nuts, which you can buy in a supermarket, are a great source of selenium.

Commas were appropriate here because we could replace them with parentheses:

Brazil nuts (which you can buy in a supermarket) are a great source of selenium.

Not using commas or parentheses would be a mistake in this case. The sentence

Brazil nuts which you can buy in a supermarket are a great source of selenium.

implies that only Brazil nuts sold in a supermarket (and not elsewhere) are a great source of selenium, which is certainly not the case.

Notice how the three examples we used at the beginning wouldn’t make sense if we put the dependent clause in parentheses:

Cars (that don’t have seat belts) aren’t allowed to carry children. (wrong)
I don’t know (which one I want). (wrong)
Could you tell me (where it is)? (wrong)

Going back to the title of this article“that” can be used only in clauses containing essential information; it is not correct to write:

Brazil nuts (that you can buy in a supermarket) are a great source of selenium.

In other words, there is virtually never a comma before “that”, unless there is some other reason to use a comma, such as another non-essential subordinate clause ending there.

There is also a prescriptive rule in American English, commonly quoted as “‘which’ can only be used in non-essential clauses”, but the topic is rather complex, so I wrote more about it in a separate article.

This article was based on my guide to the most common mistakes in English, which explains many similar topics. Why don’t you check it out?

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