Grammar Handbook: Restrictive and Nonrestrictive Clauses

The Center for Writing Studies, The University of Illinois at Urbana-Champaign

A modifying clause can be either restrictive or nonrestrictive.

:: Restrictive Clause
A restrictive modifying clause (or essential clause) is an adjective
clause that is essential to the meaning of a sentence because it
limits the thing it refers to. The meaning of the sentence would
change if the clause were deleted. Because restrictive clauses are
essential, they are not set off by commas.

All students who do their work should pass easily.

The car that I want is out of my price range.

The gas company will discontinue our service unless we
pay our bills by Friday.
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:: Nonrestrictive Clauses
A nonrestrictive modifying clause (or nonessential clause) is an
adjective clause that adds extra or nonessential information to a
sentence. The meaning of the sentence would not change if the
clause were to be omitted. Nonrestrictive modifying clauses are usually set off
by commas.

Edgar Allan Poe, who wrote "The Raven," is a great
American poet.

Puerto Rico was a Spanish colony until 1898, when it was
ceded to the United States.
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