Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
Saying just The man suddenly quickened his pace wouldn't be
sufficiently informative: The setting contains two men, so it wouldn't
be clear which one is meant. Thus, the clause who had lingered
behind acts to identify which of the two possible individuals is
meant: that is, it is restrictive.
In contrast, say the text describes a woman and a man approaching,
and mentions that the woman advances but doesn't say yet
what the man does. The reader is then told:
The man, who had lingered behind, suddenly quickened his pace.
This otherwise identical clause is now nonrestrictive. It is adding
more information about the man-that he had lingered-but it is
not serving to further identify him: He is already fully identified,
since in this case he is the only man. Here, the subject would be
as fully identified if the sentence read simply The man suddenly
quickened his pace.
In the following examples, restrictive clauses are shown in bold
type, and nonrestrictive ones in italics.
Those prisoners who had not participated in the riot had full privileges
restored.
The barber, who had not missed a day of work in fifteen years, was in his
shop as usual.
The boutique where I bought my wedding dress went out of business
last month.
The boardwalk, where the wind was strongest, was no place to be walking
that afternoon.
They left the beach when the shark arrived.
By next week, when the test results are in, I'll be able to give you a better
answer.
Customers whose numbers haven't been called yet are asked to please
wait outside.
The first person to speak up was Mr. Holland, whose weakness for lost
causes was legendary.
Usually, text will contain enough context so that the intended
meaning will come through unambiguously even if commas are not
used correctly. It is possible, however, for the comma to be the sole
source of information as to whether a clause is restrictive or not.
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