Definition: Interregnum is the period between two kings or other rulers.
The senators, who in the period of Roman kings numbered 100, were divided into 10 decuries from the Latin for 10 and the voting unit known as a curia. Each decury had a leader. Each leader was given the accoutrements of the king, his lictors and insignia, for 5 days. On the sixth day the rule was passed on to the next decury's leader. The ruling senator was called an interrex.
Later, the interregnum was used for the period between consuls, and even later, between popes.
Examples:
During the period of Rome's kings the interregnum was necessary because the kingship wasn't an hereditary office.