It had evolved from the earlier institution of Curia Regis (King's Council) and was one of the primary elements of democratic governance in the Polish dominion.

The Sejm was composed of members of the royal council or king's court (the royal court, who played the largest role), provincial crown offices such as castellans, voivodes and higher nobility or magnates (the aristocratic element represented by the senate, upper house), members of the nobility who did not hold any crown offices and city council representatives (the democratic element represented by the lower house or chamber of deputies).

The Sejm was a powerful political institution, and from early 16th century, the Polish king could not pass laws without the approval of that body.

The General Sejm of the Kingdom of Poland appeared for the first time in the years 1382–1386,[1] when nobility and city representatives began to come to the nationwide official congresses.

[5][6] The election privilege was usually limited to the most powerful nobles (magnates) or officials, and was heavily influenced by local traditions and strength of the ruler.

[8][9] Sejmiks proper date to the late 14th century when they arose from gatherings of nobility, formed for military and consultative purposes.

[24] The two chambers were: The lower house included the representatives of the major cities: Kraków, Lublin, Lwów, Poznań, Wilno, Gdańsk and Torun.

[27][28] Other locations included Brest (1653), Bydgoszcz (1520), Jędrzejów (1576), Kamień (1573), Koło (1577), Korczyn (1511), Lublin (1506, 1554, 1566, 1569), Poznań (1513), Sandomierz (1500, 1519), Toruń (1519, 1577), and Warsaw (1556, 1563, and numerous times after 1568).