Non-defining relative clauses (also known as non-restrictive, or parenthetical, clauses) provide some additional information that is not essential and may be omitted without affecting the contents of the sentence. All relative pronouns EXCEPT that can be used in non-defining clauses; however, the pronouns MAY NOT be omitted. Non-defining clauses ARE separated by commas.

The table below sums up the use of relative pronouns in non-defining clauses:

1. Relative pronoun used as a subject:

   The writer, who lives in this luxurious mansion, has just published his second novel.

2. Relative pronoun used as an object:

   The house at the end of the street, which my grandfather built, needs renovating.

3. Relative pronoun used as a possessive:
   

   William Kellogg, whose name has become a famous breakfast foods brand-name, had some weird ideas about raising children.

Some special uses of relative pronouns in non-defining clauses

1. which
   If you are referring to the previous clause as a whole, use which :

   My friend eventually decided to get divorced, which upset me a lot.

2. of whom, of which
   Use of whom for persons and of which for things or concepts after numbers and words such as most, many, some, both, none:

   I saw a lot of new people at the party, some of whom seemed familiar.
   He was always coming up with new ideas, most of which were absolutely impracticable.