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A non-restrictive clause describes a noun or pronoun, providing additional information that is not necessarily essential to the meaning of the sentence. According to the rules of grammar, non-restrictive clauses should begin with the wordswhich or who. A non-restrictive clause should also be punctuated by commas.
Usually a sentence will still make logical sense when the non-restrictive clause is removed. The non-restrictive clause offers details that help to paint a visual picture or clarify the speaker or writer's idea. Basically, however, a non-restrictive clause might be viewed as icing on the cake.
For example, “The car, which was a large SUV, was traveling so fast that Anita was frightened and jumped off the side of the road into a ditch.” If the non-restrictive clause is removed from this sentence, the meaning is still clear. It is not essential for the reader to know that the car was a large SUV in order to understand the sentence.
Choosing the correct pronoun to use with a non-restrictive clause can be confusing. Grammar rules state that a non-restrictive clause should begin with the word "which," unless the clause refers to a person, in which case it should begin with the word "who." For example, “A driver, who was traveling well above the speed limit in his SUV, frightened Anita so much that she jumped off the side of the road and into a ditch.”
Unlike non-restrictive clauses, restrictive clauses are necessary to the meaning of a sentence. Restrictive clauses not describing a person, according to the rules of English grammar, should begin with the word that. An example of a restrictive clause is, “The car that frightened Anita so much she jumped off the side of the road did not slow down.” This sentence does not make much sense if the restrictive clause is removed, because it contains too much vital information. Many people prefer to use which instead of that in a restrictive clause, and this usage is commonly accepted. For example, “The car which frightened Anita so much that she jumped off the side of the road did not slow down.”
Non-restrictive clauses in the middle of a sentence should be punctuated by two commas. For instance, “The car, which turned out to be stolen, was later found abandoned by the police.” Another example is, “Amazingly, Anita, who was barely bruised by the ordeal, continued on her walk.”