Among the words frequently used and abused are the pronouns who, which and that. Business writers often puzzle over which one to use, how to punctuate, and whether to omit altogether.  While these pronouns have a similar function in the sentence, they are used and punctuated differently. Read on to discover the correct use of relative pronouns—and some surprises!

Which and that are relative pronouns. That is, they introduce subordinate information that relates to the main clause. Yet even though they are similar in function, which and that are not interchangeable.

The word that introduces additional essential identifying information. Being essential, clauses beginning with that are not punctuated with commas. We call this use  restrictive.

Suppose you have two clocks in a room and one needs repair. To identify it, you write: The clock that is on the shelf needs repair. The clause “that is on the shelf” is essential to identify which clock. Use that and no commas.

The word which introduces additional information not essential for identification. Being nonessential, these clauses require commas. We call this use nonrestrictive.

Now, if you have only one clock in the room, and you simply want to refer to its location, you write: “The clock, which is on the shelf, needs repair.” The reference to location adds information not essential for identification. Use which with commas around the clause as the information is nonrestrictive. Note: While that is more often used to introduce an essential clause, writers sometimes choose which, especially if another t occurs in the sentence. That is a book which you must read. No commas are used.

Apply the same idea to who, a relative pronoun referring to people.
Who sometimes causes problems because it can be used in either a restrictive or a nonrestrictive sense. Let’s say two women were walking on the street. To refer to one, you might write: The woman who was carrying a heavy bag stopped to rest. The phrase “who was carrying the heavy bag” is necessary for identifying which woman you mean. Do not use commas.

If, however, a woman and a child were walking on the street, you might write: The woman, who was carrying a heavy bag, stopped to rest. Here the reader knows who stopped to rest. Identification is not an issue. You are simply adding information about what she was carrying. In this nonrestrictive sense, you need who with commas. The fact that who also takes the form whom causes confusion. Here’s a tip for correctly choosing who (subject) or whom (object): Restate the clause closely, replacing the pronoun with he or him. If he fits, pick who. If him fits, pick whom.

Ex: He is the one __we will elect.
Restate closely: We will elect him.
Equate: Him = Whom
Correct: He is the one whom we will elect.

Ex The fire chief, __ is 65, retired.
Restate closely: He is 65.
Equate:   He = Who
Correct: The fire chief, who is 65, retired.

While which refers to things and who/whom to people, that refers to people and things.
One can say “the man that I met in the elevator” or  “the project that is finishing ahead of schedule.” Both clauses are correct as long as they introduce  essential identifying  information. If the information is nonessential, use who/whom or which: “Mr. Jones, whom I met in the elevator,….” or “The marketing project, which is finishing ahead of schedule,….” When referring to a business/organization, use that or which for the entity and who for the people (“they).  

To write concise sentences, eliminate who, which and that if there’s no loss of meaning.
When the relative pronouns are simply unnecessary—and your ear will tell you when that’s the case—tighten writing by omitting:

• Simon Green, who is our department’s newest  member, will attend a meeting.
• The added piece of furniture, which was a desk, pleased the office manager.
• The pact that we signed runs for five years.

A final caution: Put the relative clause close to what it refers to in order to avoid unintended (and sometimes ridiculous) meanings:

Not: Wanted: Babysitter for 6-year-old who does not smoke or drink.

Prefer: Wanted: Babysitter who does not smoke or drink to care for 6-year-old.