Commas with Restrictive and Non-restrictive Clauses

“Who” is a relative pronoun that refers to people: “who” replaces a person’s name (e.g. Peter, Paul, Mary), a person as represented by a common noun (e.g. my doctor, our friend, an astronaut), or people as represented by a pronoun (e.g. he, she, they).  “Who” can begin both restrictive and non-restrictive clauses—groups of words that provide more information about the person whom “who” has replaced. 

Restrictive clauses provide essential information, information that is essential to the sentence and could not be removed without drastically changing the sense of the sentence.  Non-restrictive clauses provide additional information, information that could be removed and the sentence would not lose much sense.  Restrictive clauses do not need commas; non-restrictive clauses do.  The following examples show the difference.

Restrictive clause: The person who used to be the principal at my high school is now the principal at my son’s school.

If we removed “who used to be the principal at my high school” from the sentence, our readers might not understand it. For instance, “The person is now the principal at my son’s school” would leave readers wondering about the identity of “the person.” “Who used to be the principal at my high school” is therefore essential because it specifies the person for the readers.  Because the clause with “who” is essential to the sentence, we do not put commas around it.

Non-restrictive clause: Joe Peterson, who used to be the principal at my high school, is now the principal at my son’s school.

If we removed “who used to be the principal at my high school” from the sentence, our readers would still understand it:  “Joe Peterson is now the principal at my son’s school” still makes sense. “Who used to be the principal at my high school” only tells us a bit more about Joe Paterson.  Because the clause with “who” is non-essential to the sentence, we put commas around it.  (Think about non-restrictive clauses as parenthetical asides.)

Though the next two examples may appear to be identical, the commas make a significant difference when we think about them relative to the previous examples:

My wife who is 34 years old works in the Marine Biology Department.

My wife, who is 34 years old, works in the Marine Biology Department.

If “who is 34 years old” is essential to the sentence (as in the first example), I would appear to be a polygamist: in many cultures, I can only legally have one wife.  The sense of the sentence is that I have at least two wives; one of them is 34 years old.  It is more likely that “who is 34 years old” is only additional information about my one wife, and the clause should therefore be placed in commas (as in the second example).

References

Hacker, Diana.  A Writer’s Reference.  5^th ed. Boston: Bedford/St. Martin’s: 2003.

Honegger, Mark.  English Grammar for Writing.  Boston: Houghton Mifflin: 2005.

Sabin, William A. and Sheila A. O`Neill.  The Gregg Reference Manual: Third Canadian Edition. Toronto: Magraw-Hill Ryerson, 1986.