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.

The two chambers of a sejm were the Senate (senat) consisting of high ecclesiastical and secular officials, and the lower house, Chamber of Deputies [pl] (izba poselska), the sejm proper, of lower ranking officials and the representatives of all szlachta.

Both countries had centuries-long tradition of public participation in policy making, traced to the Slavic assembly known as the wiec.

[10] The first traces of large nobility meetings in the Grand Duchy of Lithuania can be found in the Treaty of Salynas of 1398 and the Union of Horodło of 1413.

It is considered that the first Seimas of the Grand Duchy of Lithuania met in Hrodna in 1445 during talks between Casimir IV Jagiellon and the Lithuanian Council of Lords.

As the Muscovite–Lithuanian Wars raged the country almost continuously between 1492 and 1582, the Grand Duke needed more tax revenues to finance the army and had to call the Seimas more frequently.

[14] Sejms, including their senate (the upper chamber), and sejmiks severely limited the king's powers.

[17][18] Later, with the rise of the magnates' power, the unanimity principle was enforced with the szlachta privilege of liberum veto (from the Latin: "I freely forbid").

[19] From the second half of the 17th century, the liberum veto was used to paralyze sejm proceedings and brought the Commonwealth to the brink of collapse.

[23] They introduced majority voting for items declared as "non crucial" (most economic and tax matters) and outlawed binding instructions from sejmiks.

After the 1569 Union of Lublin, the Kingdom of Poland was transformed into the federation of the Polish–Lithuanian Commonwealth and the numbers of sejm participants were significantly increased with the inclusion of the deputies from Lithuanian sejmiks.

[17] The two chambers were: The Constitution of May 3 specified that the deputies were elected for two years, and did not require reelection in that period if any extraordinary sejms were to be called.

[16] Due to population size differences between Lithuania and Poland, the Grand Duchy had three times less representatives than the Crown.

[27] Numbers of deputies elected to the sejm by sejmiks from particular localities, in the order of precedence, based on a 1569 decree, were as follows:[29][30]

[31][32] (The position of the Marshal of the Sejm (and sejmik) who presided over the proceedings and was elected from the body of deputies evolved in the 17th century.

[11]) Next, the kanclerz (chancellor) declared the king's intentions to both chambers, who would then debate separately till the ending ceremonies.

[35] From the end of the 16th century, the constitutions were printed, stamped with the royal seal, and sent to the chancelleries of the municipal councils of all voivodeships of the Crown and also to the Grand Duchy of Lithuania.

[18][36] Such constitutions were often subjected to some final tweaking by the royal court before being printed, although that could lead to protests among the nobility.

[45] Seen as emergency or extraordinary sessions, they relied on majority voting to speed up the discussions and ensure a legislative outcome.