RELATIVE
CLAUSES
See also Pronouns.
There are two different
types of relative clause:
- A "defining"
or identifying clause, which tells us which person or thing we
are talking about.
- A "non-defining"
or non-essential clause, which gives us more information about the
person or thing we are talking about. This kind of clause could often
be information included in brackets (...)
Example:
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:
a. the choice
of pronoun used to introduce the clause,
b. the punctuation - you must use commas with a non-defining
clause.