TANISTRY (from Gaelic Lana, lordship)  , a custom among various Celtic tribes, by which the king or chief of the clan was chosen from among the heads of the septs and elected by them in full assembly . He held office for life and was required by custom to be of full age, in possession of all his faculties and without any remarkable blemish of mind or body . At the same time, and subject to the same conditions, a tanist or next heir to the chieftaincy was elected, who if the king died or became disqualified, at once became king . Usually the king's son became tanist, but not because the system of primogeniture was in any way recognized; indeed, the only principle adopted was that the dignity of chieftainship should descend to the eldest and most worthy of the same blood . These epithets, as Hallam says, were not necessarily synonymous, but merely indicated that the preference given to seniority was to be controlled by a due regard to desert (Constit . Hist., vol. iii. c. xviii.) . This system of succession left the headship open to the ambitious, and was a frequent source of strife both in families and.between the clans . Tanistry was abolished by a legal decision in the reign of James I. and the English land system substituted .