In a dimer we have two individual polypeptide chains and these polypeptide chains can interact usually via non covalent bonds but sometimes we have covalent bonds such as disulfide bridges disulfide bonds that also hold those individual polypeptide together. Now, generally speaking, whenever we have quotinary structure those individual polypeptide chains are also known as subunits. So for example, in a dimer we have two subunits. In a trimer we have three subunits. In a tetrimer we have four subunits and so forth. Now, these subunits can be different or they can be identical.