There are two types of relative clauses, restrictive and non-restrictive. Restrictive clauses specify which (of many) nouns the speaker is referring to. They do not require commas.
- I have three brothers.
- My brother who lives in Virginia is a rocket scientist.
- My brother who lives in Ohio is a civil engineer.
- My brother who lives in Hawaii is a surfer.
Each of the relative clauses in the above examples specifies a different brother. It tells “which one.”
Non-restrictive clauses, on the other hand, are separated by commas. They merely give more information about the noun. In other words, they tell more about the same topic. The commas function as parentheses.
- I have one brother.
- My brother (who is from Oklahoma) is an architect.
- My brother, who is from Oklahoma, is an architect.