The szlachta ([ˈʂlaxta] ( listen), exonym: Nobility) was a legally privileged noble class with origins in the Kingdom of Poland. It gained considerable institutional privileges between 1333 and 1370 during the reign of King Casimir III the Great.:211 In 1413, following a series of tentative personal unions between the Grand Duchy of Lithuania and the Crown Kingdom of Poland, the existing Lithuanian nobility formally joined this class.:211 As the Polish-Lithuanian Commonwealth (1569–1795) evolved and expanded in territory, its membership grew to include the leaders of Ducal Prussia, Podolian and Ruthenian lands.
Under whos' reign did the szlachta gain institutional privileges?
King Casimir III the Great