The conditions requisite for an _interregnum_ were the non-existence of consuls, or magistrates with consular power, or a dictator. The retirement of all the other so-called patrician magistrates, i.e. _magistratus populi_, was another necessary preliminary, for the auspices could not return to the _patres_ so long as they were held, whether as _majora_ or _minora auspicia_,[600] by a patrician magistrate.[601] Hence, when a sudden occasion arose for the appointment of an interrex, it was the duty of the Senate to give notice to the patrician magistrates and to request them to retire from office.[602] The plebeian magistrates still remained in the exercise of their functions.
It was, in the later Republic, the Senate which took all further necessary action. In the early Republic there was no possibility of its being summoned, and the patrician senators met at their own discretion to appoint the interrex. But after the tribune, who was still in office, had gained the right of transacting business with the Senate, it was he who put the question, and the Senate who suggested that the _patricii_ should meet for the purpose. From this time onwards the electors felt no obligation to meet except on the suggestion of the Senate.[603]
The collegiate principle of the regal _interregnum_ and the use of the lot[604] had both disappeared; the agreement of the patrician senators took the form of the election (_creatio_)[605] of a single interrex (_prodere interregem_). This magistrate nominated his successor, as the consul nominated the dictator,[606] each succeeding interrex holding office for five days. There was no limit to the number that might be created, the interreges varying from the minimum of two to the known maximum of fourteen;[607] but there must be at least two, the first being incapable of holding the consular election, probably because he was regarded as having received the _auspicia_ irregularly. The qualifications for the interrex were, that he should be a Patrician[608] and a senator, and the instances seem to show that he was invariably chosen from the past holders of curule office.[609] The first interrex was no doubt guided by the wishes of the Senate, or of the _patres_, in the choice of his nominee, and the whole list may have been prepared before his appointment. With the creation of the highest regular patrician office, i.e. with the election of a single consul, the _interregnum_ necessarily came to an end and the interrex retired.