Non-defining relative clause

A non-defining relative clause describes a preceding noun by adding extra information about it. The noun has a clear reference (it is clear who or what we are talking about) even without the clause:

The Mona Lisa was painted by Leonardo da Vinci, who was also a prolific engineer and inventor.
The human heart, which has a mass of about 300 grams, pumps blood throughout the body.

A non-defining relative clause can also continue a story by saying what happened next:

I called my mother, who became very upset. (I called my mother, and she became very upset.)

The non-defining relative clause is separated from the rest of the sentence by commas (,). In speech, we make a short pause before and after the clause.