From the UWF Writing Lab’s 101 Grammar Mini-Lessons Series Mini-Lesson #40

 Are essential to a sentence because they restrict, or limit, the meaning of the words they modify  Not set off from the sentence with commas  The city which is known as the Mile High City has an altitude of one mile.

 Although they add important information, are not essential to the meaning of a sentence  Set off from the sentence with commas  Denver, which is known as the Mile High City, has an altitude of one mile.

Restrictive Last summer Thaun Le visited the Vietnamese village where she was born during the war. He who steals my purse steals trash. A man who thinks he has no faults has one Good things come to people who wait.

Nonrestrictive Margaret Thatcher, who was prime minister longer than anyone else in this century, made enormous changes in England. The book, old, dirty, and dilapidated, is one of my favorites. The weatherman, who on a rainy day had predicted sunshine, called the rain “Oregon mist.”