Page "Commutator subgroup" Paragraph 10

However, the product of two or more commutators need not be a commutator.

A generic example is in the free group on a, b, c, d.

It is known that the least order of a finite group for which there exists two commutators whose product is not a commutator is 96 ; in fact there are two nonisomorphic groups of order 96 with this property.