The institution of interrex existed in the Crown of the Kingdom of Poland, whose ruling classes liked to view their Commonwealth as an heir to Roman Empire traditions[citation needed].
The Commonwealth's monarch, holding a double title of the Two Nations (King of Poland and Grand Duke of Lithuania), entered into their office by free election (wolna elekcja), which often led to a relatively long interregnum.
Since 1572, the role of interrex traditionally fell to the Archbishop of Gniezno and Primate of Poland of the Roman Catholic Church.
The Archbishop could nominate a replacement (traditionally he would choose the Bishop of Kujawy).
In special circumstances he could declare war and negotiate peace.