The Imperial Diet (Latin: Dieta Imperii or Comitium Imperiale; German: Reichstag) was the deliberative body of the Holy Roman Empire. It was not a legislative body in the contemporary sense; its members envisioned it more like a central forum where it was more important to negotiate than to decide.^[1]

Its members were the Imperial Estates, divided into three colleges. The diet as a permanent, regularized institution evolved from the Hoftage (court assemblies) of the Middle Ages. From 1663 until the end of the empire in 1806, it was in permanent session at Regensburg.

All Imperial Estates enjoyed immediacy and, therefore, they had no authority above them besides the Holy Roman Emperor himself. While all the estates were entitled to a seat and vote, only the higher temporal and spiritual princes of the College of Princes enjoyed an individual vote (Virilstimme), while lesser estates such as imperial counts and imperial abbots, were merely entitled to a collective vote (Kuriatstimme) within their particular bench (Curia), as did the free imperial cities belonging to the College of Towns.^[2]

The right to vote rested essentially on a territorial entitlement, with the result that when a given prince acquired new territories through inheritance or otherwise, he also acquired their voting rights in the diet.^[3] In general, members did not attend the permanent diet at Regensburg, but sent representatives instead. The late imperial diet was in effect a permanent meeting of ambassadors between the estates.

The precise role and function of the Imperial Diet changed over the centuries, as did the Empire itself, in that the estates and separate territories gained more and more control of their own affairs at the expense of imperial power. Initially, there was neither a fixed time nor location for the Diet. It started as a convention of the dukes of the old Germanic tribes that formed the Frankish kingdom when important decisions had to be made, and was probably based on the old Germanic law whereby each leader relied on the support of his leading men. In the early and high Middle Ages these assemblies were not yet institutionalized, but were held when necessary at the decision of the king or emperor. They weren't called Diet yet, but Hoftag (court day). They were mostly held in the imperial palaces (Kaiserpfalz).

For example, already under Emperor Charlemagne during the Saxon Wars, a Hoftag, according to the Royal Frankish Annals, met at Paderborn in 777 and officially determined laws concerning the subdued Saxons and other tribes. In 803, the Frankish emperor issued the final version of the Lex Saxonum.

At the Diet of 919 in Fritzlar the dukes elected the first King of the Germans, who was a Saxon, Henry the Fowler, thus overcoming the longstanding rivalry between Franks and Saxons and laying the foundation for the German realm. After the conquest of Italy, the 1158 Diet of Roncaglia finalized four laws that would significantly alter the (never formally written) constitution of the Empire, marking the beginning of the steady decline of the central power in favour of the local dukes. The Golden Bull of 1356 cemented the concept of "territorial rule" (Landesherrschaft), the largely independent rule of the dukes over their respective territories, and also limited the number of electors to seven. The Pope, contrary to modern myth, was never involved in the electoral process but only in the process of ratification and coronation of whomever the Prince-Electors chose.

Until the late 15th century the Diet was not actually formalized as an institution. Instead, the dukes and other princes would irregularly convene at the court of the Emperor. These assemblies were usually referred to as Hoftage (from German Hof "court"). Only beginning in 1489 was the Diet called the Reichstag, and it was formally divided into several collegia ("colleges").

Initially, the two colleges were that of the prince-electors and that of the other dukes and princes. Later, the imperial cities, that is, cities that had Imperial immediacy and were oligarchic republics independent of a local ruler that were subject only to the Emperor himself, managed to be accepted as a third party.

Several attempts to reform the Empire and end its slow disintegration, notably starting with the Diet of 1495, did not have much effect. In contrast, this process was only hastened with the Peace of Westphalia of 1648, which formally bound the Emperor to accept all decisions made by the Diet, in effect depriving him of his few remaining powers. From then to its end in 1806, the Empire was not much more than a collection of largely independent states.

Probably the most famous Diets were those held in Worms in 1495, where the Imperial Reform was enacted, and 1521, where Martin Luther was banned (see Edict of Worms), the Diets of Speyer 1526 and 1529 (see Protestation at Speyer), and several in Nuremberg (Diet of Nuremberg). Only with the introduction of the Perpetual Diet of Regensburg in 1663 did the Diet permanently convene in a fixed location.

The Imperial Diet of Constance opened on 27 April 1507;^[4] it recognized the unity of the Holy Roman Empire and founded the Imperial Chamber, the empire's supreme court.

From 1489, the Diet comprised three colleges:

After the Peace of Westphalia, religious matters could no longer be decided by a majority vote of the colleges. Instead, the Reichstag would separate into Catholic and Protestant bodies, which would discuss the matter separately and then negotiate an agreement with each other, a procedure called the itio in partes.^[5] The Catholic body, or corpus catholicorum, was headed by the Archbishop-Elector of Mainz.^[6]

The Protestant body, or corpus evangelicorum, was headed by the Elector of Saxony. At meetings of the Protestant body, Saxony would introduce each topic of discussion, after which Brandenburg-Prussia and Hanover would speak, followed by the remaining states in order of size. When all the states had spoken, Saxony would weigh the votes and announce a consensus.

Frederick Augustus II, Elector of Saxony converted to Catholicism in 1697 in order to become King of Poland, but the Electorate itself remained officially Protestant and retained the directorship of the Protestant body. When the Elector's son also converted to Catholicism, Prussia and Hanover attempted to take over the directorship in 1717–1720, but without success. The Electors of Saxony would head the Protestant body until the end of the Holy Roman Empire.^[6]

After the formation of the new German Empire in 1871, the Historical Commission of the Bavarian Academy of Sciences started to collect imperial records (Reichsakten) and imperial diet records (Reichstagsakten). In 1893 the commission published the first volume. At present the years 1524–1527 and years up to 1544 are being collected and researched. A volume dealing with the 1532 Diet of Regensburg, including the peace negotiations with the Protestants in Schweinfurt and Nuremberg, by Rosemarie Aulinger of Vienna was published in 1992.

• Imperial Diet (disambiguation)