Sejm of the Polish–Lithuanian Commonwealth

The General Sejm (Polish: sejm walny, Latin: comitia generalia) was the bicameral legislature of the Polish–Lithuanian Commonwealth. It was established by the Union of Lublin in 1569 following the merger of the legislatures of the two states, the Sejm of the Kingdom of Poland and the Seimas of the Grand Duchy of Lithuania. It was one of the primary elements of the democratic governance in the Commonwealth (see Golden Liberty). The sejm was a powerful political institution that the king could not pass laws without its the approval.

The two chambers of a sejm were the Senate (senat) consisting of high ecclesiastical and secular officials, and the lower house, Chamber of Deputies (izba poselska), the sejm proper, of lower ranking officials and the representatives of all szlachta. Together with the king, the three were known as the sejming estates, or estates of the sejm (stany sejmujące, literally, "deliberating estates").

Duration and frequencies of the sejms changed over time, with the six-week sejm session convened every two years being most common. Sejm locations changed throughout history, eventually with the Commonwealth capital of Warsaw emerging as the primary location. The number of sejm deputies and senators grew over time, from about 70 senators and 50 deputies in the 15th century to about 150 senators and 200 deputies in the 18th century. Early sejms have seen mostly majority voting, but beginning in the 17th century, unanimous voting became more common, and 32 sejms were vetoed with the liberum veto provision, particularly in the first half of the 18th century. This vetoing device has been credited with significantly paralyzing the Commonwealth governance.

In addition to the regular sessions of the general sejm, in the era of electable kings, beginning in 1573, three special types of sejms (convocation, election, and coronation sejms) handled the process of the royal election in the interregnum period. In total, 173 sejms met between 1569 and 1793.