The Prince-Electors of the Holy Roman Empire were seven individuals that selected the next Holy Roman Emperor.
Comprised of three lords of the Empire (the Margrave of Brandenburg, the Duke of Saxony and the Count Palatine of the Rhine), three members of the Church (the Archbishops of Cologne, Trier and Mainz) and a neighboring independent monarch (the King of Bohemia), the Electors originally had power to select the Emperor, following the ancient Germanic tribal tradition, but in later years, it was merely a formality.
The lords represented the dukes of the nations of the Franks, Saxons, Swabians and Bavarians, while Archbishops represented the Church's most powerful Episcopal Sees in Germany.
Histories note that the electors made their selections in 1152 and 1198. In 1648, due to the Treaty of Munster, there were eight electors, which later increased to nine in 1692. By 1777, it was back down to eight. And between 1152 and 1777, due to politics and warfare, a number of electors were banned from choosing the Emperor.
In 1803, the number of electors increased to ten, removing some of the old hereditary electors and adding four new ones. But they never got to cast any votes because, in 1806, the Holy Roman Empire ended.
Electing kings is a pretty radical idea for feudal societies. It would make an interesting addition to a campaign centered on medieval political intrigue.