There were several of them because, by the Basic Law for the Empire of 1356, each Holy Roman Emperor had to hold his first diet in Nuremberg after his election. There were also a number of other diets held.
1211 elected Frederick II of Hohenstaufen emperor.
1356 Charles IV issued his 'Golden Bull' - fixing how the German emperors were elected.
Important to Protestantism (and the Turks) were the: