Which or That?

from Sacramento News & Reviews

“Which” and “that” aren’t the same word and, based on sentence structure, are called upon in different circumstances.

“That” usually precedes a restrictive clause, or information that’s essential to the sentence. “Which” is used with nonrestrictive clauses.

For example: All the decorations that aren’t green get taken down on St. Patrick’s Day. In this sentence, “that aren’t green” is vital information because only the nongreen decorations are coming down -- all green decorations will be left up. This makes it a restrictive clause.

Now, let’s try an example with “which”: All the decorations, which have been up since Christmas, will be taken down on St. Patrick’s Day. In this version, “which have been up since Christmas” is a non-restrictive clause, meaning it’s only loosely connected with the essential meaning of the sentence; it’s on-the-side-information. In this version, all the decorations, green and otherwise, are coming down.

If you’re editing a piece, think of it this way: If you see the word “which” without a comma or preposition before it, chances are it needs to be “that.”