Interrex
The interrex was literally a ruler "between kings" during the Roman Kingdom and the Roman Republic. He was in effect a short-term regent.
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 ; and from each of these decuriae one senator was nominated as decurio. Each of the ten decuriones in succession held the regal power and its badges for five days as interrex; and if no king had been appointed at the expiration of fifty days, the rotation began anew. The period during which they exercised their power was called an interregnum, and on that occasion lasted for one year, after which Numa Pompilius was elected as the new king.
After the death of each subsequent king an interrex was appointed by the senate. His function was to call a meeting of the Comitia Curiata which would elect a new king.
Under the Republic, interreges were appointed to hold the comitia for the election of the consuls when the consuls, through civil commotion or other cause such as death, had been unable to do so during their year of office. Each interrex 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, but 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 to elect the first consuls were held by Spurius Lucretius Tricipitinus as interrex was also called praefectus urbis. The interreges under the republic, at least from 482 BC, were elected from ex-consuls by the senate, 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 the senate included plebeians, 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 until the time of the Second Punic War. After that no interrex was appointed until 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 52 an interrex held the comitia in which Pompey was appointed sole consul.
List of Roman ''interreges'' (509 - 52 BC)
Unless otherwise indicated, the names and dates of the interreges are taken from Thomas Broughton's The Magistrates of the Roman Republic.| Year | Interrex | note |
| 509 | Sp. Lucretius Tricipitinus | |
| 482 | A. Sempronius Atratinus, 1st Sp. Lartius Flavus, 2nd | |
| 462 | P. Valerius Poplicola | |
| 444 | T. Quinctius Capitolinus Barbatus | |
| 420 | L. Papirius Mugillanus | |
| 413 | Q. Fabius Vibulanus | |
| 396 | L. Valerius Potitus Q. Servilius Fidenas M. Furius Camillus | |
| 391 | M. Furius Camillus II P. Cornelius Scipio L. Valerius Potitus II | |
| 389 | P. Cornelius Scipio II M. Furius Camillus III | |
| 387 | M. Manlius Capitolinus Ser. Sulpicius Camerinus L. Valerius Potitus III | |
| 355 | Q. Servilius Ahala I & II M. Fabius Ambustus I & II Cn. Manlius Capitolinus Imperiosus C. Fabius Ambustus C. Sulpicius Peticus L. Aemilius Mamercinus | Servilius & M. Fabius appointed twice |
| 352 | 11 unknown interreges L. Cornelius Scipio | Cornelius as the twelfth of a series of interreges |
| 351 | C. Sulpicius Peticus II M. Fabius Ambustus III | |
| 340 | M. Valerius Corvus M. Fabius Ambustus IV? or M. Fabius Dursuo | |
| 332 | 4 unknown interreges M. Valerius Corvus II | Valerius as the fifth and last of a series of interreges |
| 326 | 14 unknown interreges L. Aemilius Mamercinus Privernas | Aemilius as the fifteenth of a series of interreges |
| 320 | Q. Fabius Maximus Rullianus M. Valerius Corvus III | |
| 298 | Ap. Claudius Caecus P. Sulpicius Saverrio | |
| 291 | L. Postumius Megellus | |
| 222 | Q. Fabius Maximus Verrucosus? | Fabius was twice Interrex, both at unknown dates. This is one possible date as suggested by Broughton. |
| 216 | C. Claudius Centho P. Cornelius Scipio Asina | Scipio held the comitia that elected the consul Varro |
| 208? | Q. Fabius Maximus Verrucosus II? | Mommsen and Broughton suggests this as a possible date for Fabius as interrex. Livy instead attributes the elections to the Dictator, T. Manlius Torquatus |
| 82 | L. Valerius Flaccus | |
| 55 | ||
| 53 | ||
| 52 |