Here's why:
* Nonrestrictive clauses provide additional information about a noun or pronoun but are not essential to understanding the meaning of the sentence. They are set off by commas.
* Restrictive clauses, on the other hand, are essential to understanding the meaning of the sentence. They are not set off by commas.
Example:
* Nonrestrictive clause: "The cat, which was very fluffy, sat on the windowsill." (The information about the cat being fluffy is extra, and the sentence would still make sense without it.)
* Restrictive clause: "The cat that ate the bird is now hiding under the bed." (The information about the cat eating the bird is essential to identifying which cat is being discussed.)
Key characteristics of a nonrestrictive clause:
* It provides additional information.
* It is not essential to the meaning of the sentence.
* It is set off by commas.
Let me know if you have any further questions!