Commas with Nonessential Elements

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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.

Clue: 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	vs.	Essential
The average world temperature, however, has continued to rise significantly. (word) Company managers, seeking higher profits, hired temporary workers to replace full-time staff. (phrase) My uncle, who is eighty years old, walks three miles every day. (clause)		The sixth-century philosopher Boethius was arrested, tortured, and bludgeoned to death. (word) The person checking tickets at the counter asked for a form of identification. (phrase) 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 handout on verbals at http://owl.english.purdue.edu/handouts/grammar/g_verbals.html (handout #5) which includes information on participial phrases, as well as the handout on appositives at http://owl.english.purdue.edu/handouts/grammar/g_appos.html (handout #71) which covers appositive phrases. Both of these documents address the essential/nonessential distinction for these kinds of phrases.

You can try three different exercises that allow you to practice these rules, each with its own answer key.

Exercise #1 at http://owl.english.purdue.edu/handouts/grammar/g_commaessEX1.html
Exercise #2 at http://owl.english.purdue.edu/handouts/grammar/g_commaessEX2.html
Exercise #3 at http://owl.english.purdue.edu/handouts/grammar/g_commaessEX3.html

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