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 (Consul. 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 andbetween the clans. Tanistry was abolished by a legal decision in the reign of James I and the English land system substituted.