The reasons for the existence of Crow-Omaha terminologies have long been debated because of difficulties in associating them with specific features of social organization or practice. However, by going back to the theories of earlier authorities like Josef Kohler, Radcliffe-Brown and Lévi-Strauss and using them to interpret some key ethnographies, it is possible to suggest why they exist. That is, from ego’s perspective, the vertical equations that define Crow-Omaha terminologies unite descent lines whose members in each generation can substitute for one another in relations with ego’s descent line, as marriage and other ties between those lines work themselves out over a time scale of, very often, several decades. Ultimately this can be linked to the long claimed links between Crow-Omaha terminologies and the prohibition of certain kin types in marriage, which typically act to delay the repetition of previous marriage alliances for one or more generations. It is suggested that Crow-Omaha terminologies have less to do with the prohibitions themselves than with these periods of delay.