That vs. Which

To understand when to use “that” or “which,” you will first need to understand what a clause is. A clause is a group of words that contains a subject and a verb.

There are two kinds of clauses: independent and dependent. An independent clause can stand alone as a sentence.

For example:

I would love to see a movie.

A dependent clause cannot stand alone; thus it depends on the addition of an independent clause for it to make sense.

For example:

That interests me. (Dependent clause)

I would love to see a movie that interests me. (Join dependent clause to an independent clause.)

“That” and “which” are relative pronouns that introduce relative (or adjective) dependent clauses.

“That” is a relative pronoun used to begin restrictive (essential) clauses that are necessary to the meaning of a sentence.

In the example above, the dependent clause is necessary to the meaning of the sentence because I don’t want to see just any movie; I want to see a movie that interests me. For clauses beginning with “that,” do not use a comma in front of them because commas are not used around essential clauses.

“Which” is a relative pronoun used to begin nonessential relative clauses. In other words, these clauses are not necessary to the meaning of the sentence. Use commas around nonessential elements in a sentence.

For example:

The ancient oak tree, which shaded the west side of my house, was felled by the hurricane.

The independent clause can stand by itself:

The ancient oak tree was felled by the hurricane.

The dependent clause (which shaded the west side of my house) has commas around it because it is not necessary to the meaning of the sentence.

Sometimes the nonessential clause comes at the end of the sentence. In this case only one comma is needed.

For example:

For a healthy glow, use our Illuminating Face Mask, which removes impurities and the appearance of fine lines.

The main point of the sentence is the first part of it:

For a healthy glow, use our Illuminating Face Mask. This can stand alone.

The last part of the sentence (which removes impurities and the appearance of fine lines) is not necessary to the main point of the sentence, so it has commas around it.

To review, “that” is used to begin a dependent clause that cannot be removed from the sentence without changing its meaning. “Which” is used to begin a dependent clause that can be removed without changing the main point of the sentence.