Knowing when to use ‘that’ and when to use ‘which’ in certain sentences can be confusing—in fact, getting the two mixed up is an error that even experienced and effective writers make. So what’s the difference between the two, and how can you tell which is the correct word to use? It doesn’t have to be as complicated as some people might have you believe, and this week’s post explains the simple rules you can remember to make the right ‘that’ or ‘which’ choice. First, it’s important to understand what part of a sentence requires you to choose between ‘that’ and ‘which’: a clause. A clause is a unit of words that either forms part of a sentence, or functions as a sentence by itself. It usually features both a subject (what the sentence is about) and a predicate (whatever is being said about the subject). For example: Subject Predicate That article was very informative. ‘That’ and ‘which’ are both used to introduce certain types of clause: restrictive or defining clauses, and non-restrictive or non-defining clauses. Restrictive or defining clauses add information that is essential to a sentence’s meaning. ‘That’ should always precede a restrictive or defining clause. For example: Restrictive/defining clause The flight that I booked online was surprisingly cheap. Non-restrictive or non-defining clauses add information that is not essential to a sentence’s meaning. ‘Which’ should always precede a non-restrictive or non-defining clause, and should always be preceded by a comma. For example: Non-restrictive/non-defining clause The flight, which I booked online, was surprisingly cheap. The reason for using ‘that’ and ‘which’ in different ways is because they can alter a sentence’s meaning. In the first example given above, the use of ‘that’ suggests that more than one flight was booked, but the one referred to is the one booked online. Without the restrictive clause, we have no way of knowing which flight was the cheap one. However, in the second example, the use of ‘which’ suggests that only one flight was booked. That it was booked online is incidental information, because if the non-restrictive clause is removed, the sentence still makes sense: we still know that the flight was cheap. The easiest way to figure out whether you need to use ‘that’ or ‘which’ is to remove the part of the sentence you’re preceding with either word and see if it still has the same meaning. If it does, the part you’ve removed is a non-restrictive clause and needs a ‘which’; if it changes the meaning, the part you’ve removed is a restrictive clause and needs a ‘that’. For example, take the sentence ‘the books, which are hardcover, are on sale’. If all the books are on sale, the fact that they’re hardcover is just extra information, so the meaning doesn’t change if you remove the ‘which’ clause. However, if only some of the books are on sale—the hardcover ones—you need a ‘that’ instead (‘the books that are hardcover are on sale’). The only time the rules differ is when you’re using ‘which’ with a preposition such as ‘in’ or ‘to’, which generally occurs in a defining clause and doesn’t require a comma—for example, ‘the room in which the conference was held’ or ‘the seminar to which the students were going’. ‘That’ and ‘which’ are little words that can make a big difference to your meaning if they’re used incorrectly. Once you know their simple usage rules, you’ll always make the right choice!