nonrestrictive phrases and clauses

A nonrestrictive phrase or clause is one that adds information but is not essential to the meaning of the sentence.

The principal investigator, who has studied thin films for 10 years, will chair the panel discussion.

The passive solar features, which were suggested by NREL staff, reduced the agency's energy bills by 30%.

Nonrestrictive or nonessential phrases and clauses are enclosed between two commas when the phrase or clause is within a sentence, and they usually begin with "which" rather than "that." See also restrictive phrases and clauses and which and that.

Subject:

Grammar and Usage