Interrex

The Interrex (plural interreges) was literally a ruler "between kings" (Latin inter reges) 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 (groups of ten); 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.