This post addresses three issues. Do you use a comma with a descriptive clause? Do you or don’t you use a comma with “who”? When do you use “that” and “which”?
Let’s start with a definition of restrictive clauses. A restrictive clause tells which person or thing you are writing about. It points out one person or thing from a group or category so the reader knows which one you mean. For example, in the sentence
Where are the shoes that I bought yesterday?
the expression “that I bought yesterday” tells you which shoes I’m writing about. Of all the shoes (the category), I want to restrict your attention to the ones I bought yesterday. The expression “that I bought yesterday” provides essential information without which the reader won’t know which shoes I’m writing about.
Because that information is essential to understand the meaning of the sentence, it isn’t separated from the rest of the sentence with commas. Also, notice that the clause begins with “that,” not “which.” (Use “who” for people.)
On the other hand, a non-restrictive clause is not essential information. A non-restrictive clause doesn’t tell the reader which person or thing you are writing about. If you use a non-restrictive clause, the reader already knows which person or thing. The non-restrictive clause provides additional information about that person or thing. For example, in the sentence
Susan has a great car, which is a Honda Accord.
the expression “which is a Honda Accord” provides an explanation or description of Susan’s great car. Based on this sentence, Susan only has one car, and “which is a Honda Accord” provides additional information about it.
Because that information is not essential to understand the meaning of the sentence, it is separated from the rest of the sentence with commas. Also notice that the clause begins with “which,” not “that.” (Use “who” for people.)
Summary of the information so far
Begins with “that” or “who”
Provides essential information to know which person or thing
Don’t use commas
Begins with “which” or “who”
Does not provide essential information; provides a supplemental description
So far so good?
Restrictive and non-restrictive clauses with “who.”
You’ll notice that both restrictive and non-restrictive clauses may begin with “who.” In sentences using “who,” the comma, or lack of comma, determines the meaning of the sentence! Let’s look at two examples with very different meanings.
1. I met the governor who was drinking whiskey. (restrictive)
Imagine that I just returned from a meeting of governors. More than one governor was present. I want the reader to know which one I met. One of the governors was drinking whiskey, and that’s the one I met.
“Who was drinking whiskey” is essential information to identify the governor. It restricts the reader’s attention from the category or group called governors to one particular governor. Because it is required to understand my meaning, the clause is not separated with commas.
2. I met the governor, who was drinking whiskey. (non-restrictive)
Imagine that I just returned from the state capital. While there, I met the governor. The reader knows which governor I am writing about. “The governor” in this case can only be one person. The expression “who was drinking whiskey” provides the reader with an additional description of the governor. It doesn’t tell the reader which one; the reader already knows which one.
This non-restrictive clause can’t restrict the category or group of governors to a particular governor because the category or group called governor only has one person in it. Because it is not required to understand my meaning, the clause is separated with commas.
Let’s quickly look at two more.
1. The clerk who took my order was very nice. (restrictive)
2. The clerk, who took my order, was very nice. (non-restrictive)
In the first example, more than one clerk was present. The expression “who took my order” tells the reader which clerk was nice.
In the second example, only one clerk was present. The expression “who took my order” isn’t needed to tell the reader which clerk took my order. Instead, it provides some additional but non-essential information about that clerk.
A poser for you
Which sentence indicates that the businessman has only one daughter?
1. The businessman brought a present for his daughter, who had been sad.
2. The businessman brought a present for his daughter who had been sad.
In one sentence (#1), the businessman only has one daughter. She had been sad. In the other sentence (#2), the businessman has more than one daughter. The sad one got the present. The comma, therefore, makes a difference in how many daughters the businessman has.
Summary of information about “who”
Provides essential information to know which person
Don’t use commas
Is not needed to know which person
Comma rules from Zen Comma
Rule U: Use commas to separate non-restrictive phrases and clauses starting with “who.”
Rule V: Don’t use commas to separate restrictive phrases and clauses starting with “who.”
Are you a person who uses commas correctly?
Need help with commas? Get Zen Comma, an instructive reference guide on the 17 major uses and misuses of commas, available in PDF and Kindle formats. Read more about Zen Comma.