Broadview Guide to Writing
Use of the Comma 2: Essential and Non-Essential Elements

When a comma is used to set off an introductory clause or phrase in a sentence, one comma on its own is used. When commas are used to set off transitional or parenthetical elements in the middle of a sentence, a pair of commas is used—one before the transitional or parenthetical phrase, and one at the end of it. When commas are used to set off the items in a list, several may be used (including a comma before the last item in the series).

When is it appropriate to set off material with commas on the grounds that it is parenthetical? The short answer is that we set off material as parenthetical when the material is not essential to our understanding of the sentence. Let's look at an example:

 Police have captured the suspect, who was traveling on a German passport.

Here the implication is that there is only one suspect. The fact that he was traveling on a German passport is merely additional information; it is not essential to our identifying who is being referred to. Such parenthetical information should be set off by commas.

 Police have captured the suspect who was traveling on a German passport. Two other suspects remain at large.

Here the information regarding the passport is essential if we are to identify which of the suspects is being referred to. The clause "who was traveling on a German passport" restricts the meaning of the sentence to one of the three suspects in particular. In such cases no commas are used to set off the essential information.

Again, restrictive (or essential) sentence elements are words or phrases that follow nouns (or noun phrases) and that are essential to their meaning. Essentials do not take commas. Non-restrictive (or non-essential) elements are words or phrases that follow nouns (or noun phrases) and that provide more information about the nouns. Non-essentials take commas before and after them.

For related exercises see "Restrictive and Non-Restrictive Modifiers" under Parts of Sentences.

For each of the following, indicate which option is correctly punctuated.

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