Interrex

Interrex is a 'ruler between kings'. He is a specific type of regent.

Originally, they were officers of the Roman Republic. When no consuls were elected by New Year's Day, for whatever reason (civil strife, filibustering, etc.), a patrician senator would be elected (or appointed) by the Senate to serve as interrex for five days. After that, another interrex would be elected, also a patrician senator for five days. The second interrex would be responsible for organizing new elections. If no elections were held still, then new interreges would continue to be appointed every five days until they were held.

The interrex was also a position of the Polish-Lithuanian Commonwealth. In the Commonwealth, kings were elected, which often led to a relatively long interregnum perriod. Traditionally the role of interrex was fulfilled by a primate.

de:Interrex