Subunits are the individual polypeptide chains. Each subunit is given a designation Greek letter (alpha, beta, etc.) A protein with several subunits is an oligomer.

i.e. a protein made with two of the same subunits could be (alpha)2 or (beta)2. while a protein with two subunits that are different could be (alpha)(beta).

*Note this differs from the alpha/beta 3D formations in secondary protein structure. The Greek alphabet being referred to in quaternary structure involves the subunit being identical to another subunit in the same protein and designating it with a Greek alphabet letter. A subunit that is different from another subunit in the same protein will have a different Greek letter designated to it.