A non-restrictive clause is a clause which is not needed to identify the word it modifies, i.e., it is just additional information. As a non-restrictive clause is not essential to the meaning of a sentence, it is offset with commas. For example:
You are not limited to commas when offsetting a non-restrictive clause. You can parentheses (brackets) or dashes too.
Read more about your choices of punctuation for offsetting a non-restrictive clause.
Examples of Non-restrictive ClausesHere are some more examples of non-restrictive clauses:
Some More Examples of Non-restrictive ClausesHere are some more examples of non-restrictive clauses in real-life quotes (non-restrictive clauses shaded):
What is a clause?
What is a modifier?
What are restrictive clauses?
More about using commas with which, that, and who
More about your choice of parentheses
Glossary of grammatical terms
Click on the one with a non-restrictive clause in bold:
IF PARENTHESES (BRACKETS) SEEM OKAY, THEN USE COMMAS
Writers are sometimes unsure whether to use a comma(s) with a clause starting with which or who.
If you are happy to offset your clause with parentheses (brackets), then use commas. This tip works because non-restrictive clauses (the ones with commas) just provide additional information. They're not essential.
Read more about your choice of parentheses.
COMMAS WILL AFFECT THE MEANING
The meaning of your sentence will be affected by your decision whether to use commas around a clause. Both examples below are grammatically sound, but they have different meanings.