Commas with That and Which

I’ll answer two questions at the same time. Do you use that or which? Do you need commas with that and which?

To answer both questions, you need to understand restrictive and non-restrictive phrases and clauses. Here are the answers in brief.

1. Use which and commas with non-restrictive phrases and clauses.
2. Use that and no commas with restrictive phrases and clauses.

Now, let find out why.

Restrictive phrases and clauses: A restrictive phrase or clause points out which thing you are writing about.

Let’s say you have four filing cabinets in your office and that all but one cabinet is locked. The unlocked cabinet is the one next to the window. You need someone to come and lock the cabinet because you don’t have a key.

You decide to send an e-mail to the maintenance office. You need to tell the maintenance officer which cabinet is unlocked. You correctly write this statement:

“Please come to my office as soon as possible and lock the cabinet that is next to the window.” (The restrictive phrase is underlined.)