Incompatibilitas (Latin, 'incompatibility') was a principle instituted in the Kingdom of Poland (later, from 1569, in the Polish–Lithuanian Commonwealth), which forbade an individual to hold two or more official administrative positions. The principle of incompatibilitas evolved in the 15th-16th centuries in response to a demand from middle and lesser nobility (Szlachta), and it was designed to curtail the sway of more powerful high nobility/aristocracy/plutocracy (the Magnates).
The specific acts of law that constituted the incompatibilitas rule were bans on holding:
Commonwealth ministers were the equivalents of modern central-government officials. There were 10 officials (5 for Poland, 5 for Lithuania): a Great Crown Marshal, Great Lithuanian Marshal, Great Crown Chancellor, Great Lithuanian Chancellor, Crown Deputy Chancellor, Lithuanian Deputy Chancellor, Great Crown Treasurer, Great Lithuanian Treasurer, Court Crown Marshal and Court Lithuanian Marshal. The Court Marshals were considered subordinate to the Great Marshals. Lithuanian ministers, while enjoying the same powers as Crown ministers, were considered to be lower in the hierarchy. Hetmans were also considered "ministers" but had no seat in the Senat.