Sejm of the Polish–Lithuanian Commonwealth

Timeline

1. 1565–1568Piotrków Trybunalski · PiotrkówIn the Sejms of the years before the Union of Lublin, deputies debated plans for uniting the two states at Piotrków and elsewhere, discussing seat allocation, legal unification, and terms for the elective monarchy, laying procedural and political groundwork for the federal parliament of 1569.
2. 1569Lublin · LublinThe Union of Lublin of July 1569 merged the Kingdom of Poland and the Grand Duchy of Lithuania into a commonwealth; the Sejm was expanded into a joint parliament of both states, Lithuanian nobles were admitted to the Senate, and seats in the Chamber of Deputies were redistributed, forming a unified legislative and taxing body.
3. 1570Warsaw · WarsawThe first Sejm convened under the full federal system after the union addressed taxation, border defense, and noble privileges, establishing that Polish and Lithuanian deputies voted under the same rules; the practice of the nobles' liberum veto also took shape during this period.