When should a comma be used to set off nonessential information?

A comma should be used to set off nonessential information when it provides additional details or clarifications about a subject but can be removed from the sentence without changing its overall meaning or structure. Nonessential information, also known as nonrestrictive or nondefining information, is not crucial to the sentence and can be omitted without affecting the main idea.

Here are some instances when a comma should be used to set off nonessential information:

1. Appositives: When a noun or noun phrase is placed next to another noun to provide additional information or rename it, a comma should be used to set off the appositive. For example: "My friend, Sarah, is an excellent painter."

2. Parenthetical expressions: When a word, phrase, or clause is inserted into a sentence to provide extra information or an aside, it should be set off by commas. For example: "The weather, as you may have noticed, is quite unpredictable."

3. Relative clauses: When a relative clause provides additional information about a noun or pronoun, it should be separated by commas. For example: "The book, which was written by a famous author, became an instant bestseller."

4. Introductory phrases or clauses: When a phrase or clause is used at the beginning of a sentence to provide context or introduce the main idea, it should be followed by a comma. For example: "After finishing his work, John went for a walk."

5. Nonessential adjectives: When multiple adjectives are used to describe a noun, a comma should be used to separate nonessential adjectives from the essential ones. For example: "She wore a beautiful, silk dress to the party."

It is important to note that if the information is essential to the meaning of the sentence, it should not be set off by commas. Essential information, also known as restrictive or defining information, is necessary for understanding the sentence and should not be removed.