A workable classification system must comprise a hierarchy in which groups are contained completely within larger composite groups with no overlap. The human species, for example, belongs to the genus Homo, which is a member of the primate order, which is in the class Mammalia and so on.
Two classification systems – the phonetic and the phylogenetic – are feasible. In the former, each group in the hierarchy is determined entirely by similarities of structure. In the letter, the groupings are based on the pattern of their evolutionary history.