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Some modifying elements of a sentence are essential, restricting the meaning of a modified term, while others are nonessential and don't restrict the modified term's meaning. These nonessential elements, which can be words, phrases, or clauses, are set off with commas.
Rule: Use commas before and after nonessential words, phrases, and clauses, that is, elements embedded in the sentence that interrupt it without changing the essential meaning.
If you leave out the element or put it somewhere else in the sentence, does the essential meaning of the sentence change? If so, the element is essential; if not, it is nonessential.
Nonessential: The average world temperature, however, has continued to rise significantly. (word)Essential: The sixth-century philosopher Boethius was arrested, tortured, and bludgeoned to death. (word)
Nonessential: Company managers, seeking higher profits, hired temporary workers to replace full-time staff. (phrase)
Essential: The person checking tickets at the counter asked for a form of identification. (phrase) Nonessential: My uncle, who is eighty years old, walks three miles every day. (clause) Essential: The woman who interviewed you is my sister. (clause)Deciding whether an element is essential or nonessential can sometimes be tricky. For help identifying two common types of phrases that can be either essential or nonessential, see the OWL handouts on verbals, which includes information on participial phrases, as well as the handout on appositives, which covers appositive phrases. Both of these documents address the essential/nonessential distinction for these kinds of phrases.
You can try three different interactive exercises that allow you to practice these rules, each with its own answer key.
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