Defining vs. Non-Defining Relative Clauses
Bob's mum has lost her keys.
Bob's mum, who is a musician, has lost her car keys. (NON)
My friend Jane moved to Canada. (I only have one friend named Jane)
My friend Jane, who is a scientist, moved to Canada. (NON)
Tamara has two cats. Both of them are black.
Tamara's two cats, which can play outside, are black. (NON)