Sejm gradually evolved from a meeting of the most powerful magnates to a full legislative institution representing all of the nobility.
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 Sejm more frequently.
[1] As the importance of Sejm grew, nobles from more distant regions started electing representatives from their districts and sending them to the meeting.
[3] However, gradually the lesser nobles understood that the Sejm gave them power to block new taxes and by mid-16th century they started to demand more privileges for themselves.
When the separate Sejm for Lithuania was officially abolished in 1569, it adopted the name of Lithuanian Convocations and continued to meet until the partitions of the Polish–Lithuanian Commonwealth in 1795.
It debated matters concerning the Grand Duchy of Lithuania or tried to establish a common position among Lithuanian delegates before departing for the Sejm of the Commonwealth.
[1] The convocations retained basic structures (upper and lower houses) and procedures of the Sejm: each powiat could send only two representatives.