A Way With Words: Which and That
Just as there are rules for when to use “who” and “whom,” and “farther” and “further,” there are also rules for when to use “that” and “which.”
Take these two sentences:
My to-read book pile that has four books is on my night stand.
My to-read book pile, which has four books, is on my night stand.
In the first sentence, the inference is that there is are more than one pile of books, and the one with four books is the one on the night stand.
The second sentence refers to one pile, and it just happens to have four books.
You use “that” if the clause is necessary to the sentence, if it limits what the clause is referencing. In the first sentence, the clause describes a specific stack of books – one that has four books.
If you omit the extra phrase, and it changes the sentence’s meaning or makes it less precise, then it is a restrictive clause.
Take out the reference to the number of books, and the sentence can apply to any pile of books. This sentence is about a specific stack of four books.
If the clause can be removed, and is unnecessary to the meaning of the sentence, then you use “which.” A non-restrictive clause is also separated with commas.
The second sentence refers to a single pile of books that just happens to be made of four books.
Here are other examples:
My first car that was red was a Mustang.
My first car, which was red, was a Mustang.
The first sentence is specifically about my first red car. If my actual first car was an Accord, but my first red car was a Mustang, removing “that was red,” makes the sentence less precise.
The second sentence is about my first car. The phrase, “which was red” is extra information. It could have just as easily read, “which was blue.”
Use “which” to include additional, but nonessential information, but use “that” for important details.