The Diets of Central Europe can be distinguished from provincial estates, such as those found in France, by the very nature of “Estate law,” which was the basis of their powers. If power in France was “no more divisible than the point in geometry,” things were quite different in other European states. From Spain to Poland, Estate law, consisting of a series of privileges, formed a kind of customary Constitution in which power was shared between representatives of “nations,” generally the nobility, and the sovereign. Diets thus had an essential role in the government of the state, although the historiography, hypnotized by the notion of absolutism, has seen them solely as the remainder of a continually weakening power.

The powers enjoyed by Central European Diets evolved according to chronologies that varied greatly according to the country. Weakened during the seventeenth century in the Hapsburgs hereditary lands, the Diet remained powerful in Hungary, always representing a potential political danger for the sovereign (until the nineteenth century), as well as a potential collaborating body for the nation and the king (see the Diet of 1741). In the Holy Roman Empire, although it is often asserted that the institutions of the Empire lost power after 1648, the Diet nevertheless remained an important centre for the Empire and even for Europe: when it settled at Regensburg, it was seen as a permanent diplomatic congress. In the Imperial estates, the evolution of Diets was highly variable. For example in Saxony, the power of the Diet was relatively important during the seventeenth century: it broke the will of John George I, and contributed to military and economic decision-making. It took until the reign of Augustus the Strong for the sovereign, who was now more comfortable financially, to reduce its powers. The same dynamic of reducing the power of estates can be seen in the Estates of Brandenburg, with differing fortunes. Poland, finally, represents a remarkable counter-example to the sovereign's increase in power. The Polish Diet continued to expand its competence during the seventeenth century, granting its members the most extensive of rights, as illustrated by the famous liberum veto. The situation that resulted was nevertheless disastrous for the country, and contributed to the deadlock of Polish political institutions during the eighteenth century.

The Diet did not meet of its own initiative, but was convened by the sovereign at a frequency imposed by law, and was more or less well-respected depending on the relations between the sovereign and the nobility. The assembly was consequently convened much more regularly in Bohemia than in Hungary. The duration of a Diet varied greatly based on the session, going from a few weeks to six or nine months in normal times, and even much longer in periods of crisis.

The power of Diets was very tightly linked to that of the nobility, which constituted its majority. They were often divided into chambers or curia. While composition varied from one estate to another, four political orders were nonetheless generally present:

- the prelates, who could be from the secular or regular clergy, depending on their respective wealth: due to Ottoman occupation, Hussite wars, and the Reformation, the regular clergy in Hungary or Bohemia lost a large part of its land base, unlike in Lower Austria, where it remained very powerful;
-the lords or magnates (in Hungary): the major noble families of Central Europe enjoyed much greater power than the Grands du royaume of France. The seigneuries were immense and entirely subject to the authority of the lord, and practically inaccessible to the power of the sovereign. The group of lords often dwarfed the Diet with its authority. The lords and prelates formed­—wherever they happened to exist, in Hungary or Poland for example­—the upper chamber of the Diet (the Senate in Poland);
- the nobility (knights, simple gentlemen) provided most of the Diet's members, although with important disparities according to the territory. In Bohemia, the nobility was less and less powerful both demographically and economically, while it remained very numerous in Hungary or Poland. During the early modern period, the Diet no longer admitted all nobles, with the exception of summoned Diets (election of the sovereign) in Poland. They therefore elected representatives who sat for them;
- the cities: in Hapsburg territories, as well as in a certain number of the Imperial estates of the Holy Roman Empire, royal cities had a seat in the Diet, although their role grew increasingly weaker. In Hungary, cities rounded out the assembly. For example, the lower chamber of the Diet was held by the nobility.

There were nevertheless exceptions to this composition, of which we will present two examples.

The Imperial Diet (Reichstag) was not a representative assembly, but rather a forum of Imperial Estates. As such, seats in it were ensured by law, although not all estates were represented (the Netherlands, Italian estates, Hapsburg territories, and Swiss cantons were absent, as were knights). It consisted of three colleges: electors, princes (ecclesiastical, secular and counts) and free cities.

The Diet of Transylvania included three orders that were different from the preceding ones, for it involved three nations that constituted the political forces of the principality: the Hungarian, Székely, and Saxon nations.

The remit of Diets differed according to the territory, period, or power relations between the sovereign and the orders: it was in favour of the former in the Hapsburg hereditary lands, in a state of tense compromise in Hungary, and in favour of the nobility in Poland. The Diet lost the power to elect the sovereign in most estates over the course of the seventeenth century (Bohemia 1627, Hungary 1687, Transylvania with incorporation into Hungary for example), with the exception of Poland. The Diet retained a powerful legislative role in certain territories, such as Poland, Hungary, Transylvania, or in the case of the Imperial Diet. Yet it was sometimes the sovereign who had the initiative and set the agenda (Imperial Diet or the Diet of Hungary in the eighteenth century, for example).

The Diets voted on the taxes granted to the sovereign, especially for financing military spending. They also had a religious competence, that of assisting the sovereign with the Catholic reconquest. However in Hungary, the lower chamber, which for a long time consisted of numerous Protestants, also saw to it that the liberties granted to Hungarian Protestants were respected, and that the Imperial Diet served as a guarantor of the rights conferred by the Treaty of Westphalia. Finally, with regard to economic matters, the Diets often had a highly protectionist role. The Imperial Diet also had a diplomatic role, as it voted over war or peace for the Holy Roman Empire.

In order to ensure the proper functioning of its duties, a more or less substantial administration could exist in connection with Diets: permanent commissions meeting between sessions, or offices for the execution of decisions (Bohemia and Moravia for example).

Diets were a genuine centre of political life in Central Europe, despite the disparities that existed between their respective powers. Their financial competence gave them a voice in the sovereign's policies, notably with respect to foreign policy.