Non-defining Relative Clauses Definition

A non-defining relative clause gives extra information about a noun or noun phrase and has commas at both ends:

My sister, who lives in France, is coming to stay with me next week. ('who lives in France' is not essential, which means that I only have one sister and she does not need to be defined by the relative clause)

'Who' and 'whose' are used for people. 'Which' and 'whose' are used for things. 'That' cannot be used in a non-defining relative clause.