Who, Whom, Which, and That

by Cherie Tucker

There are two rules concerning these words when they are used to introduce descriptive information in a sentence.  First, who and whom refer to people; which and that refer to things.

He is the man who sold us the bridge.
We have your signature, which closes the deal.
These are the times that try men’s souls.

Second, some constructions using these words require a comma, but some do not.  The comma alters the meaning, so you must decide what it is you wish to say.  If the information introduced by who/m is essential to the meaning of the sentence, there should be NO comma.  Test by leaving that part out.

He only dates women who can dance.
He only dates women.

Since the who clause determines the description of “women” in the sentence, there should be no comma.  By contrast, if the clause is merely informational, it must be set off by commas.

The women, who had been standing vigil since noon, finally went home.

In this sentence, the fact that the women were standing vigil is not essential to the message that they went home.  The commas are necessary, acting like parentheses.

With that and which, the comma use is easier to determine.  Generally, that introduces essential information, and which introduces non-essential.

The book that changed her life had been banned.
The book, which they later banned, changed her life.

In the first example, that restricts the meaning of “book” to only one book, the life-changing one.  In the second example, the fact the book was banned is incidental to the change in her life and must be set off by commas.

There are times when you can leave these little words out, but only if the omission doesn’t lead to misreading.  In the sentence, “We knew that Bill was guilty,” it seems logical that you could leave that out.  However, if the first part appears at a page turn, you have “We knew Bill,” which isn’t what you are saying.  If your sentence reads, “We knew that he was guilty,” then leave thatout.  Since no one over the age of three says “We knew he,” that omission is a safe one.

Cherie Tucker, owner of GrammarWorks, has taught writing basics to professionals since 1987, presenting at the PNWA conference.  She currently teaches Practical Grammar for Editors at the University of Washington’s Editing Certification program and edits as well.  GrammarWorks@msn.com