Nonrestrictive clauses are typically set off by commas. A nonrestrictive clause provides extra information that can be omitted without changing the essential meaning of the sentence. For example:
- My brother, who lives in New York, is visiting us next week.
In this sentence, "who lives in New York" is a nonrestrictive clause. If you remove it, the main meaning of the sentence remains intact: "My brother is visiting us next week."
In addition to commas, parentheses or dashes can also be used to set off nonrestrictive clauses, but commas are the most common punctuation for this purpose. Here’s how it looks with the other forms:
- My brother (who lives in New York) is visiting us next week.
- My brother — who lives in New York — is visiting us next week.
However, the most standard and widely accepted punctuation in formal writing to set off nonrestrictive clauses is commas.