On 3 May 1505 King Alexander I Jagiellon granted the Act of "Nihil novi nisi commune consensu" (Latin: "I accept nothing new except by common consent"). This forbade the king to pass any new law without the consent of the representatives of the nobility, in Sejm and Senat assembled, and thus greatly strengthened the nobility's political position. Basically, this act transferred legislative power from the king to the Sejm. This date commonly marks the beginning of the First Rzeczpospolita, the period of a szlachta-run "Commonwealth".
The king's power was checked by what group?
the nobility