Interrex or "inter-rex" (Latin; plural, interreges) was literally a ruler "between kings." He was in effect a short-term regent.
The interreges agreed among themselves who should be proposed as king and if the senate approved of their choice, they summoned the assembly of the curiae, and proposed the person whom they had previously agreed upon; the power of the curiae was confined to accepting or rejecting him. The decree of the curiae, by which they accepted the king, was called jussus populi .After the king had been elected, the curiae conferred the imperium upon him by a special law, lex curiata de imperio.
Interreges were appointed under the republic for holding the comitia for the election of the consuls, when the consuls, through civil commotions or other causes, had been unable to do so in their year of office. Each held the office for only five days, as under the kings. The comitia were, as a general rule, not held by the first interrex; more usually by the second or third ; but in one instance we read of an eleventh, and in another of a fourteenth interrex. The comitia for electing the first consuls were held by Sp. Lucretius as interrex was also called praefectus urbis. The interreges under the republic, at least from B.C. 482, were elected by the senate from the whole body, and were not confined to the decem primi or ten chief senators as under the kings. Plebeians, however, were not admissible to this office; and consequently when plebeians were admitted into the senate, the patrician senators met together without the plebeian members to elect an interrex. For this reason, as well as on account of the influence which the interrex exerted in the election of the magistrates, we find that the tribunes of the plebs were strongly opposed to the appointment of an interrex. The interrex had jurisdictio.
Interreges continued to be appointed occasionally till the time of the second Punic war, but after that time we read of no interrex, till the senate, by command of Sulla, created an interrex to hold the comitia for his election as Dictator, B.C. 82. In B.C. 55 another interrex was appointed to hold the comitia, in which Pompey and Crassus were elected consuls and we also read of interreges in B.C. 53 and 52, in the latter of which years an interrex held the comitia, in which Pompey was appointed sole consul.