Spot the commas! Restrictive and non-restrictive relative clauses

In writing, punctuation generally distinguishes non-restrictive relative clauses (e.g. 1 and 2 below) from restrictive relative clauses (e.g. 3 and 4). The former, as parenthetical adjuncts, are usually surrounded by commas or brackets, whereas restrictive relative clauses are not:

1 How can the French, who invented joie de vivre, the three-tier cheese trolley and Dior’s jaunty New Look, be so resolutely miserable?

2 Mikhail Kalashnikov, who was in his 20s when he created the AK-47 just after the Second World War, died in his home city of Izhevsk.

3 Actually, calling Mandela a hero falls woefully short in adequately portraying the man who fought apartheid…and changed the politicallandscape of his country.

4 Offstage, the dancer who once had a reputation for enjoying himself behind the scenes has finally been called to heel.

In speech, we have to rely on intonation to distinguish the two types of relative clause. In cases of doubt, try the ‘incidentally’ or ‘by the way’ test: non-restrictive relatives generally sound natural if either is inserted after the relative pronoun, whereas restrictive relatives do not.