Non-defining relative clause
A non-defining relative clause (/ˌnɒndəˈfaɪnɪŋ(g) ˈrelətɪv ˈklɔ:z/), also known as a non-identifying relative clause, or non-restrictive relative clause is a relative clause that acts as an adjectival, adding secondary information to a sentence, almost as an afterthought. Commas are used to "isolate" the clause from the rest of the sentence and if the clause is removed, the sentence remains well-formed.
- My brother, who is an engineer, has been working in the same company for over twenty-five years.
- My favourite dish is roast beef and Yorkshire pudding, which is difficult to find on a restaurant menu.
that cannot be used
- John, who (not that) plays rugby and cricket, is always busy at weekends. [this is a bad example]
- Paul took me for a drive in his new car, which ('not' that) he had bought last week. [this is a bad example]
'who or which cannot be omitted
- We stayed at the Grand Hotel, which Ann had recommended to us.
This is one of the few cases that whom can be used (when it is the object):
- This morning I met Diane, who (or whom) I hadn’t seen since Christmas.
- My brother (who is an engineer) has been working in the same company for over twenty-five years.