Relative clauses can be broken down into two types: (1) restrictive and (2) non-restrictive.
A restrictive clause provides information that is pertinent or essential to the meaning of the sentence.
A non-restrictive clause, also called a parenthetical or an appositive, contains information that is extra. Information contained in a non-restrictive clause may be removed from the sentence altogether without changing the overall meaning of the sentence. Because this information is not crucial to the meaning of a sentence, it is set apart with commas, or, more emphatically, with
dashes, or parentheses.
Both kinds of clauses can be signaled by the use of wh word (which, when, where, and who). That is used to signal a restrictive clause. Because a restrictive clause contributes to the basic meaning of the sentence, it is not separated from the rest of the sentence with commas or other surrounding
The meaning of a sentence can be altered by whether a clause is punctuated so as to show that it is a restrictive clause (giving crucial information) or a non-restrictive clause (giving extra information).
Here are some examples:
The woman whom I met at the baseball game was wearing my same shirt.
The guy that my old roommate married is boring.
I had the answer that made the most sense.
The piece of chocolate cake, which probably has a million calories in it, was delicious.
Charles Dickensm, who was great writer in the late 19th Century, is the author of Little Dorrit.
He reads and studies maps for fun, which is no exaggeration.