RESTRICTIVE AND NON-RESTRICTIVE ELEMENTS
Restrictive elements (also called essential elements) and nonrestrictive elements (also called nonessential elements) function as modifiers in sentences. A modifier is a word or group of words that describes or limits other words, phrases, and clauses.
Restrictive elements are essential to the meaning of a sentence.
The man who wore the yellow hat waved to us from the tour bus.
[This information identifies this man in particular and sets him
apart from other men on the same bus.]
The woman sitting on the beach under the umbrella was reading
Tolstoy and eating a popsicle. [This information is essential
because it identifies which woman on the beach was doing this.]
Restrictive elements are not set off by commas.
Nonrestrictive elements are not essential to the meaning of a sentence.
Miss Trevor, with whom I'd graduated from high school, is a successful
businesswoman. [This information is unnecessary to the reader's
understanding that Miss Trevor is successful.]
The envelope, which had had no return address, was yet sitting
on the table at the foot of the staircase. [It is unnecessary for the
reader to know that the envelope had no return address to
acknowledge its location in the house.]
Nonrestrictive elements are set off by commas.