Restrictive and Non-restrictive clause: Usage of Which, that and who

Which or That?

"That" restricts the reader's thought. This introduces a specific bit of information to complete a sentence's meaning. "Which" is non-restrictive and introduces incidental rather than essential information to the meaning of the sentence. "Essential" does not mean that the information is not important or relevant, just that it's not essential to properly understand central intention of the sentence.

That

The officers can eat the box of donuts that is in the squad room.

To paraphrase (and elaborate)

There are two boxes of donuts. Only the one in the squad should be eaten. Eating the box in the chief's office will get you fired.

Which

The officers can eat the box of donuts, which is in the squad room.

To paraphrase (and elaborate)

The Department has a box of donuts for the officers. It's in the squad room.

These sentences are a little awkward but clearly demonstrate the distinction in the use of "that" and "which." Let's try a little more natural sentence.

That

The protective vests that have been distributed to the detective bureau are the new lighter models.

To paraphrase (and elaborate)

The entire Department received new protective vests. The detective bureau received the new lighter models.

Which

The protective vests, which have been distributed to the detective bureau, are the new lighter models.

To paraphrase (and elaborate)

Detective bureau received new protective vests. They are the new lighter model.

Summary:

Restrictive clauses ("that") modify, focus and limit. Because the information they supply is essential to the intended meaning of the sentence, they are not set off by commas.

Non-restrictive clauses ("which") do not limit the words they modify. They simply add information that otherwise would not be provided. Non-restrictive clauses are set off by commas because the information they provide is supplementary, not essential to the meaning of the sentence.

Who

Restrictive and non-restrictive clauses need not begin with "that" and "which." For example, if humans are being discussed, "who" may become the appropriate choice for both kinds of clauses. In such cases, a comma all by itself can transform the meaning of the sentence.

"Who" (restrictive)

The officer helped the civilians who had been shot.

Note: there is no comma before "who." Therefore, what follows is a restrictive clause. Not all of the civilians had been shot.

"Who" (non-restrictive)

The officer helped the civilians, who had been shot.

Note: there is a comma before "who." Therefore, what follows a non-restrictive clause. It also changes the sentence to mean that all the civilians had been shot.

Restrictive and Non-restrictive Clause

Restrictive and non-restrictive elements are not always introduced by relative pronouns (without "who," "which," or "that"). Sometimes they simply are treated as restrictive or non-restrictive elements. In such cases, the comma is crucial to the meaning.

Non-restrictive

My brother, Joe, works for the FBI.

The addition of the non-restrictive element here merely provides us with the name of my brother. Because I have only one brother, his name is not essential to the reader in identifying him. Therefore, it makes it non-restrictive or non-essential to the meaning of the sentence and is set off by a comma.

Non-restrictive

My brother Joe works for the FBI.

The omission of the comma creates a restrictive meaning. Now we are implying that I have more than one brother. One of them is named "Joe."