Tanistry was a system for passing on titles and lands. In this system the Tanist (Irish Tánaiste; Scottish Gaelic Tà naiste; Manx Tanishtagh) was the office of heir-apparent, or second-in-command, among the (royal) Gaelic patrilineal dynasties of Ireland, Scotland and Man, to succeed to the chieftainship or to the kingship.
Origins
The Tanist was chosen from among the heads of the roydammna or "righdamhna" (literally, those of kingly material) or alternatively among all males of the sept in question, and elected by them in full assembly. The eligibility was based on patrilineal relationship, which meant the electing body and the eligibles were agnates with each other; actually the composition and the governance of the clan were built upon male-line descent from a common ancestor. The office existed since the dawn of recorded history in Ireland, and probably greatly pre-dates it. For example, a story concerning Cormac mac Airt lists his eldest son as his Tanist. Following his death