Setting Off Non-Essential Material

Sometimes certain modifying words, phrases, or clauses, although they may contribute to the meaning of the sentence, are not essential to its meaning. That is, deleting the material would not substantially change the sentence’s meaning or emphasis. Commas should set off such non-essential material.

Non-essential	The President, knowing the need for drastic action, declared the entire state a disaster.
Essential	Anyone knowing the condition of the region will approve the President’s action.

1. Nonrestrictive modifiers

Modifying phrases or clauses may be restrictive or nonrestrictive. Clauses and phrases that modify the noun but are not essential to understanding the meaning of the noun within its sentence are called nonrestrictive. That is, they are said not to restrict the meaning. Such clauses and phrases must be set off from the rest of the sentence with commas:

Compare these two sentences:

Actors who have big egos are often insecure.

Actors, who have big egos, are often insecure.

At first glance, the two sentences above seem to mean exactly the same thing. Upon closer examination, however, you can see that they convey different meanings. Look at the first sentence again.

Actors who have big egos are often insecure.

In this sentence, who have big egos is restrictive; the writer’s intention is to limit the nouns actors only to those who have big egos. The sentence suggests that only those actors with big egos – not all actors – are insecure. The modifying phrase restricts the meaning of actors, the noun it modifies, and therefore it is not set off by commas.

Now look at the second sentence again.

Actors, who have big egos, are often insecure.

In this sentence, the modifying phrase who have big egos is nonrestrictive because it suggests that all actors have big egos. Because the modifying phrase does not restrict the meaning of the noun actors, it is set off by commas.

Note: Adjective clauses following a proper (capitalized) noun will nearly always be nonrestrictive and thus require commas:

Elizabeth Blackwell, who attended medical school in the 1840s, was the first American woman to earn a medical degree.

Many clauses and phrases, however, are essential to understanding the meaning of the noun – they narrow the meaning of the word or word group they modify – and are not separated by commas. These are called restrictive and are not set off with commas.

The glory which is built upon a lie soon becomes a most unpleasant encumbrance. (Mark Twain)

Note: A name that follows a common noun or noun phrase is restrictive and should not be set off by commas:

The philosopher Alfred North Whitehead once wrote that the history of philosophy was a series of footnotes to Plato.

But when the name comes first, the common noun that follows it is nonrestrictive and should be set off by commas:

Francois Truffaut, the film director, died of cancer in 1984.

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