I’ll be the first to admit it: A lot of grammar rules seem so nitpicky or obscure that it takes a lot of motivation to learn them. (When was the last time you had to name the dative or ablative case in a sentence, anyway?) But if there’s one rule that’s worth committing to memory, it’s when to use “which” and “that.”  Writer, here’s a quick guide to using these tricky relative pronouns.

First, a little on relative pronouns: These parts of speech introduce relative clauses, which are dependent clauses that modify a word or idea in the sentence’s main clause. Aside from which and that, other common relative pronouns include who/whom, whoever/whomever and whose. (More on how to use those pairs correctly in upcoming Write In Color posts).

There are two types of relative clauses, restrictive and non-restrictive. Understanding these two is your key to solving the which vs. that riddle. Here’s the general rule to keep lodged in the grammar section of your brain:

If a clause is restrictive, meaning the information in it is essential to the sentence, use “that,” and don’t separate the clause with commas.

Shapes that have four equal sides are called squares.

Remove the bit about the four equal sides, and the sentence reads, “Shapes are called squares.” And our inner geometry prowess tells us that’s just not true. Here are a few more examples:

The book that inspired me to become a writer is Charlotte’s Web.

Restaurants that sell burgers and fries are good for the taste buds, not the waistline.

If a clause is non-restrictive, it can be removed from the sentence without changing its meaning. This is when you use commas and a relative pronoun such as “which.”

My apartment, which is a bit of a dump, doesn’t have air conditioning.

Removing the relative clause may keep you from knowing just how subpar my apartment is, but it doesn’t change the meaning of the main clause, which is about its lack of AC. Two more examples:

The banjo, which is my favorite instrument to play, can have four or five strings.

That slice of cake, which I wolfed down within seconds, was the best dessert I’ve had in a long time.

One more thing: Remember that “that” and “which” never refer to people. If your clause is modifying a person, use “who.”  (i.e. The man who I love is 30 years old, not The man that I love is 30 years old.)

And with that, you’re set to conquer that and which.

- Emily Hubbell