Alternating copolymers are commonest copolymers. Comment 

Explanation

Alternating copolymers are usually produced when two monomers which differ in their reactive groups undergo condensation. A dicarboxylic acid reacts with a diol and the chain is propagated to give a polyester like polyethylene terephthalate (PET).
PET also called dacron or terylene is an alternating copolymer. Nylon 6,6 is another familiar condensation alternating copolymer. Buna type rubbers, on the other hand, are addition alternating copolymers.