Usually, the terms restrictive and nonrestrictive describe relative clauses—clauses beginning with who, which, or that.

A clause is restrictive (or essential or identifying) if the meaning of the sentence would not be clear without it. Do not use a comma with a restrictive clause.

Consider this example:

Full-time students who attend summer school usually graduate in less than four years.

Without the modifying clause, the meaning of this sentence would change, as not all full-time students graduate in less than four years. Consequently, the relative clause is restrictive and does not require commas.

A clause is nonrestrictive (or nonessential or non-identifying) if the meaning of the sentence would remain the same even without it. Always use commas with nonrestrictive clauses.

The following sentence contains a nonrestrictive clause:

College students, who buy many compact discs, are a main source of income for record companies.

The relative clause in this sentence simply adds extra information. The basic meaning of the sentence would not change if the clause were omitted. Consequently, the clause is nonrestrictive and needs to be enclosed by commas.

When a relative clause follows a proper name, the clause is always nonrestrictive because the name already makes the identity clear. Consequently, relative clauses following proper names are always set off by commas, as shown in the following examples:

Dudley Wilson, who was the highest scorer on our team, received a scholarship offer from Penn State.
Penn State, which recruits only the best players, was impressed with Dudley Wilson’s scoring record.

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