Szlachta
The first "free election" (Polish: "wolna elekcja") of a king took place in 1492. (To be sure, some earlier Polish kings had been elected with help from bodies such as that which put Casimir II on the throne, thereby setting a precedent for free elections.) Only senators voted in the 1492 free election, which was won by John I Albert. For the duration of the Jagiellonian Dynasty, only members of that royal family were considered for election; later, there would be no restrictions on the choice of candidates.

Q: What dynasty was in reign during the election of 1492?
Jagiellonian Dynasty