The Golden Bull of 1356 was a decree issued by a Reichstag in Nuremberg headed by Emperor Charles IV, Holy Roman Emperor (see Diet of Nuremberg) that fixed, for a period of more than four hundred years, an important aspect of the constitutional structure of the Holy Roman Empire.
The Golden Bull explicitly named the seven Kurfürsten (Electors) who were to choose the King of the Germans, who would then usually be crowned Holy Roman Emperor by the Pope later. Consequently, the Bull speaks of the rex in imperatorem promovendus, the "king to be promoted emperor" -- although the distinction between the two titles would become increasingly irrelevant (and virtually nonexistant after 1508).
Even though the practice of election had existed earlier and most of the dukes named in the Golden Bull were involved in the election, and although the practice had mostly been written down in an earlier document from 1338, the Golden Bull was more precise in several ways. For one, the dukeships of the Electors were declared indivisible, and succession was regulated for them to ensure that the votes would never split. Secondly, the Bull prescribed that four votes would always suffice to elect the new King; as a result, three Electors could no longer block the election, and the principle of majority voting was explicitly stated for the first time in the Empire. Finally, the Bull cemented a number of privileges for the Kurfürsten to confirm their elevated role in the Empire. It is therefore also a milestone in the establishment of largely independent states in the Empire, a process to be concluded only centuries later.