There are two different types of relative clause:
- A “defining” or identifying clause, which tells us which person or thing we are talking about. This kind of clause could often be information included in brackets (…)
- A “non-defining” or non-essential clause, which gives us more information about the person or thing we are talking about.
- The farmer (his name was Fred) sold us some potatoes.
- The farmer, whose name was Fred, sold us some potatoes.
It is important to see the difference between the two types of clause, as it affects:
- the choice of pronoun used to introduce the clause,
- the punctuation – you must use commas with a non-defining clause.