On 3rd May 1791, the Sejm passed a basic law which was the result of a social, political and systemic compromise. Although its formal name was the Government Act (at the time, the word "government" was used to mean the political system, the organization of state authority), it went down in history as the Third of May Constitution, and was the modern world's second constitution after the American Constitution of 1787.

However, it still contained some feudal and class elements akin to "class-oriented constitutions" like the Polish King Henry's Articles of 1573, the cardinal laws of 1768, or the Swedish regirungsforms of 1720, 1772 and 1789. The Polish basic law was inspired above all by the political and social thinking of the European Enlightenment. It was linked to the model of the American Constitution of 1787, to the work of the French Constituent Assembly which produced the Declaration of Human and Civil Rights of 26th August 1789 and the Constitution of 3rd September 1791, and also to the system in Great Britain, where a parliamentary monarchy was taking shape. It also took account of Polish tradition.

The Constitution opens with an introduction setting out its aims. The main body of the text is divided into eleven synthetic Articles: I. The Dominant Religion; II. The Landed Gentry; III. Towns and Townspeople; IV. The Peasantry; V. The Government, i.e. the appointment of public authorities; VI. The Sejm, i.e. the legislative authority; VII. The King, the executive power; VIII. The Judiciary; IX. The Regency; X. The Education of the Royal Children; XI. The National Armed Forces.

It should be emphasised how logical and lucid the Government Act was. Its first part, concerning the social system, consists of three articles (II-IV), corresponding to the social classes in Poland at the time. The second part, concerning the political system outlined in Article V, divides the authorities into legislative (Article VI), executive (Article VII) and judicial (Article VIII). The vast range of issues covered here made it necessary to deal with the problems of the regency, the education of the heirs to the throne and the armed forces in separate Articles. The Law on Towns of 18th April 1791 is an integral part of the Constitution.

In terms of the social order, the Government Act, although it preserved the class structure of the state, tried to make a compromise between the nobility and the bourgeoisie. It was linked, if only in part, to the memorial presented to the king and the Sejm on 2nd December 1789 by the delegates to the royal cities' congress (also known as the Black Procession). The bourgeoisie, who were influenced to some extent by events in France, demanded the same rights and freedoms as only the nobility had previously enjoyed, as well as a share of political power (representation in the Sejm and the right to hold public office).

The Law on Royal Cities extended the privilege of personal immunity neminem captivabimus nisi iure victum to middle-class property owners, re-organised the cities and made them self-governing, and repealed the regulations forbidding the nobility to engage in trade and commerce in the cities. The bourgeoisie were allowed to take public office and to become army officers. Twenty-four municipal representatives were admitted to the House of Deputies, although they had no decisive voice. They did, however, serve in the executive branch: on state commissions, in the police and the treasury and in assessors' courts. The middle classes were allowed to buy landed estates and it was made much easier for them to enter the ranks of the nobility.

There were small changes in the peasants' circumstances. Article IV of the Constitution opened up more possibilities for change and reform, placing the peasants under the protection of the "law and government of the country". The landed gentry were encouraged to sign agreements with their serfs defining their obligations. Both sides' observance of these agreements was to be guaranteed by law.

The Constitution confirmed the nobility's dominance of the country's political life, by guaranteeing them all their previous rights and privileges.

In terms of political reform, the Government Act was also the result of personal compromise and various political trends and programmes. Its authors were King Stanisław August Poniatowski, a great admirer of the English constitutional monarchy; Ignacy Potocki, the Lithuanian grand marshal, who strove to ensure that the state was dominated by an efficient and sovereign Sejm of the nobility; and Hugo Kołłątaj, who favoured a compromise between the nobility and the bourgeoisie and a continuation of Poland's "gentle revolution".

Article V of the Government Act based the system of government on Rousseau's doctrine of the sovereignty of the nation, and Montesquieu's idea of dividing the authorities into legislative, executive and judicial. The Constitution uses the term "nation" to mean different things. It sometimes uses it to mean the "nation of nobles," but in Article XI in particular, which deals with the nation's armed forces, the word nation is used to mean the entire Polish society.

The principle of division of authority was not followed consistently. The "Sejm, or the legislative body" took definite first place and, within it, the House of Deputies was the "repository of national supremacy". The Sejm was to be in a state of "constant readiness," which meant that Deputies held seats for two years and could be called upon to attend extraordinary sittings, if the need arose. The "liberum veto" was abolished, and all issues were to be decided by a majority vote. Deputies' instructions--orders issued to them by local assemblies on how to act at parliamentary sittings--were declared invalid. Deputies were henceforth representatives of the whole nation. The formation of confederations was also banned. After the death of King Stanisław August Poniatowski, members of the Senate and House of Deputies were to be elected. The king appointed one life senator from two candidates chosen by the local assembly. The king was now merely the leader of the Senate, without the right of legislative sanction and no longer one of the three estates sitting in the Sejm.

Executive power was vested in the king and the newly established Legal Guard. The monarchy was to be hereditary from then onwards, and was to pass to the Saxon Wettin dynasty after King Stanisław August's death. As in England, the king was no longer responsible for his actions: ministers countersigning his decisions took the responsibility for him. The king chaired the Legal Guard, which was made up of the Primate, as chairman of the National Education Commission, and five ministers appointed by the king: the ministers of the treasury, the army, the police, foreign affairs and seals. The collective ministerial bodies elected by the Sejm the commissions for education, the treasury, the army and the police, were subordinate to the Legal Guard. These commissions were chaired by ministers who were not its members. They were constitutionally and parliamentarily answerable to the Sejm: if they broke the law they could, at the joint request of both Houses, be tried by the Sejm Court, and they had to leave office by order of the Sejm. Local administration was in the hands of civil and military law and order commissions, elected by the local assemblies.

Independent courts were the judicial authority. Article VIII of the Constitution maintained the class judicial system: gentry's courts and tribunals for the nobility, municipal courts for the bourgeoisie, and referendary and manor courts for the peasantry.

The Third of May Constitution was in force for just over a year. Overthrown by foreign intervention and Polish traitors, it remained, after the fall of the independent Republic, during the years of partition, the symbol of the struggle to regain Poland's sovereignty. In the words of its authors, Potocki and Kołłątaj, it was "the last will of the dying Fatherland".