An interregnum is a period between kings. In particular:
In some monarchies, such as the United Kingdom, an interregnum is usually avoided due to a rule described as "the king died, long live the king", i.e. the heir to the throne becomes a new monarch immediately on his predecessor's death or abdication. This is not so in other monarchies where the new monarch's reign begins only with coronation or some other formal or traditional event. In the Polish-Lithuanian Commonwealth for instance, kings were elected, which often led to relatively long interregna. During that time it was the Polish primate who served as an interrex (ruler between kings).