The office of interrex was supposedly created following the death of Rome's first king Romulus, and thus its origin is obscured by legend. The Senate of the Roman Kingdom was at first unable to choose a new king. For the purpose of continuing the government of the city, the senate, which then consisted of one hundred members, was divided into ten decuriae (groups of ten); and from each of these decuriae one senator was nominated as decurio. Each of the ten decurios enjoyed in succession the regal power and its badges for five days as interrex; and if no king was appointed at the expiration of fifty days, the rotation began anew. The period during which they exercised their power was called an interregnum and at that time lasted for one year, after which Numa Pompilius was elected as the new king.1
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 such as death, 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, who was originally the curio maximus; 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.clarification needed 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. After that no interrex was appointed till the senate, by command of Sulla, named L. Valerius Flaccus to hold the comitia for his election as Dictator in 82 BC. In 55 BC another interrex was appointed to hold the comitia in which Pompey and Crassus were elected consuls. There were interreges in 53 and 52 BC, in the latter of which an interrex held the comitia in which Pompey was appointed sole consul.