Initially all the so-called "great ones of the Empire" (Große des Reiches) were entitled to vote, but by the second half of the 13th century, only the prince electors were entitled to participate in the royal election.
Literature
Ulrich Reuling (1979), Die Kur in Deutschland und Frankreich: Veröffentlichungen des Max-Planck-Instituts für Geschichte (in German), Göttingen: Vandenhoeck & Ruprecht, ISBN978-3-525-35376-9