A review of previous hypotheses on the meaning of tanaise rig and a new interpretation of the term. Tanaise (rig) in the law tracts, narrative literature and indigenous chronicles is analysed to disprove the standard interpretation as ‘tanist’, that is, an heir designate selected during the lifetime of a reigning king, and to argue instead that tanaise rig in earlier medieval Ireland is the representative of a king or his second in command. This analysis also establishes that tanistry began to be practised in Ireland—and then not widely—only in the later middle ages, although it existed as an ideal in the earlier middle ages.