ACADEMIC SKILLS PROGRAM
STUDENT SERVICES AND DEVELOPMENT
Commas and Essential and Non-Essential Grammatical Units
To explain this use of commas, I often put these two sentences on the blackboard:

Dan gave flowers to his girlfriend, who lives in Sudbury.

Dan gave flowers to his girlfriend who lives in Sudbury.
I then ask the students in the class which sentence is correct. In other words, should a
comma be inserted in this sentence? Invariably half the class answers that the first
sentence is correct, and the other half of the class answers that the second is correct.
What do you think?
In fact, both sentences are correct, but they have very different meanings. I can tell from
the first sentence Dan really loves his girlfriend and is faithful to her, but the second
sentence tells me that Dan is a Romeo; that he has at least one other girlfriend, if not
many, and that they live in other cities!
Isn’t it strange and interesting that a little comma can tell so much about someone’s love
life! How does it do that?
One of the uses of a comma (for others, please check out the handout/link on commas) is
to separate non-essential information from essential information. Some grammar books
call essential information “restrictive” and non-essential “non-restrictive.”
In our revealing sentences above, we learn that Dan is faithful in the first sentence
because if he has one girlfriend then the information about where she lives is incidental,
or non-essential. Therefore, we can see that when information is not necessary for the
identification or understanding of the main clause, that information can be set aside, as it
ACADEMIC SKILLS PROGRAM
STUDENT SERVICES AND DEVELOPMENT
were, with a comma. Notice that in the second sentence, there is no comma. This means
that the information about where his girlfriend lives is essential; we would not understand
which girlfriend was getting flowers unless we knew where she lived. Of course, the
only time we would have such a need would be if Dan had more than one girlfiends!
Let’s try a few more examples:

Students, who are hard-working, should expect much from their education.

Students who are hard-working should expect much from their education.
Once again, both sentences are perfectly correct but have very different meanings. The
first sentence tells us that all students should expect much from their education, and oh,
by the way, students are hard-working. The clause “who are hard-working” is nonessential. The second sentence is telling us that “students who are hard working” are the
students who should expect much from their education, the inference being that students
who are not hard working should not expect much!
So far, these examples are of sentences which work either with commas or without, the
meanings affected but the sentences still acceptable. However, most sentences are not so
flexible when it comes to essential or non-essential units.
For example:

Vegetables, stir-fried in a wok, are uniquely crisp and flavourful.

Vegetables stir-fried in a wok are uniquely crisp and flavourful.
Notice that putting in commas around a non-essential phrase“stir-fried in a wok” means
that the writer is actually saying that all vegetables are uniquely crisp and flavourful.
Unfortunately, this is not always true. Our second sentence is the correct sentence
because without the commas the writer is saying that vegetables must be stir-fried in a
wok in order for them to be uniuqely crisp and flavourful.
ACADEMIC SKILLS PROGRAM
STUDENT SERVICES AND DEVELOPMENT
One more example:

“Silent Night,” now being sung by the NorthBay Children’s choir, is my favorite
Christmas carol.
Here is an example of a sentence with a non-essential phrase “now being sung by the
North Bay Children’s choir.” As much as we might appreciate the information that the
carol is being sung by the children’s choir, the really important message, the main
message, is that this particular carol is the writer’s favorite. It is highly unlikely that the
writer would dislike the song if it was sung by any other group, a statement of such being
created if the commas were left out and the phrase became essential.