But if the king’s power was legally free from restraint, and we do not believe that there was any large body of _leges_ binding his authority, it could not have been free from the limitations imposed by custom and constitutional usage. Customary law securing rights for the people is said to have been raised to the level of positive law by Servius Tullius.[231] But even the earlier usages must have formed a kind of code—such a code as that which contained the pontifical ordinances known as the _leges regiae_.[232] It was the belief in the existence of this early customary law which led to the later description of the king’s power as an _imperium legitimum_.[233] Amongst his constitutional obligations was that of consulting the Senate in any important matter. There can be little doubt that the original council of elders (_senatus_) was a body of nominees selected by the king as his permanent advising body (_consilium publicum_).[234] In consequence the position of senator could not have been a life-office; there could neither have been any definite mode of attaining the dignity, nor any claim on the part of an individual to retain it. A new king might decline to summon some of his predecessor’s councillors; he might even, perhaps, change the _personnel_ of his advisers during the course of his reign. It was in later times believed that the freedom of selection was so great that no stigma attached to members who were “passed over” (_praeteriti_) by the king.[235] Yet tradition attributes a definiteness to the Senate which is not consistent with the idea of a purely arbitrary selection. Its numbers at any given time are fixed, and it is to some extent made representative of the whole patrician community—for an increase in the number of full burgesses involved a corresponding increase in the numbers of this body.[236] The number, originally 100,[237] was raised by successive steps to 300 before the close of the monarchy.[238] The two obvious units of representation were the _curiae_ and the clans; but the latter, from their larger numbers, formed a better basis for reflecting the opinion of the whole community, and tradition does seem to have imposed a kind of constitutional necessity on the king of distributing his councillors as evenly as possible amongst the patrician _gentes_.[239] It was thus that the distinction between the older and the newer clans was perpetuated in the procedure of the Senate;[240] but the clan-influence left its strongest mark by giving a name to the members of the body itself. It was the leading heads of families (_patres familias seniores_) that the king summoned; and, in asking their advice, he addressed them as “heads of houses” (_patres_).