8.86 punctuation

A non-defining relative clause usually has a comma in front of it and a comma after it, unless it is at the end of a sentence, in which case you just put a full stop. Dashes are sometimes used instead of commas.

• Sir Denis, who is 78, has let it be known that much of his collection is to be left to the nation.

You never put a comma or a dash in front of a defining relative clause.

• The woman who owns this cabin will come back in the autumn.