Use a comma to set off a nonrestrictive element. A restrictive element restricts the meaning of the word it modifies and is therefore essential to the meaning of the sentence. It is not set off by commas. Example: The children needed shoes that were sturdy. In this sentence we learn that the children needed a specific type of shoes; therefore the clause “that were sturdy” is essential to the sentence and does not have a comma. The word that is used with restrictive clauses. A nonrestrictive element describes a word whose meaning is already clear without the additional words. It is not essential to the meaning of the sentence and is set off with commas. Example: The children needed sturdy shoes, which were expensive. In this sentence we learn an extra fact—the shoes were expensive. That phrase merely adds more information, and it needs a comma because it is not vital to the meaning of the sentence. The word which is used with unrestrictive clauses. . Elements that may be restrictive or nonrestrictive include: adjective clauses, adjective phrases, and appositives. An adjective clause usually follows the noun or pronoun it describes and begins with a relative pronoun (who, whom, whose, which, that) or a relative adverb (when, where). Restrictive Example: A Corporation that has government contracts must maintain careful personnel records. Non-Restrictive Example: The U. S. Coast Survey, which was established in 1807, was the first scientific agency in this country. An adjective phrase is a prepositional or verbal phrase that functions as an adjective. Restrictive Example: One corner of the attic was filled with newspapers dating from the turn of the century. Non-Restrictive Example: The helicopter, with its 100,000-candlepower spotlight illuminating the area, circled above. Appositives are nouns or pronouns that rename a nearby noun. Restrictive Example: Rick James’ song “Fire It Up” was blasted out of amplifiers ten feet tall. Non-Restrictive Example: Darwin’s most important book, On the Origin of Species, was the result of many years of research.