Incompatibilitas was a principle instituted in the Kingdom of Poland

Kingdom of Poland (1385–1569)

The Kingdom of Poland of the Jagiellons was the Polish state created by the accession of Jogaila , Grand Duke of Lithuania, to the Polish throne in 1386. The Union of Krewo or Krėva Act, united Poland and Lithuania under the rule of a single monarch...

 (later, from 1569, in the Polish-Lithuanian Commonwealth

Polish-Lithuanian Commonwealth

The Polish–Lithuanian Commonwealth was a dualistic state of Poland and Lithuania ruled by a common monarch. It was the largest and one of the most populous countries of 16th- and 17th‑century Europe with some and a multi-ethnic population of 11 million at its peak in the early 17th century...

) which forbade an individual to hold two or more official administrative

Public administration

Public Administration houses the implementation of government policy and an academic discipline that studies this implementation and that prepares civil servants for this work. As a "field of inquiry with a diverse scope" its "fundamental goal.....

 positions. The principle of incompatibilitas evolved in the 15th-16th centuries in response to a demand from middle and lesser nobility (Szlachta

Szlachta

The szlachta was a legally privileged noble class with origins in the Kingdom of Poland. It gained considerable institutional privileges during the 1333-1370 reign of Casimir the Great. In 1413, following a series of tentative personal unions between the Grand Duchy of Lithuania and the Kingdom of...

), and was designed to curtail the sway of more powerful high nobility/aristocracy

Aristocracy

Aristocracy , is a form of government in which a few elite citizens rule. The term derives from the Greek aristokratia, meaning "rule of the best". In origin in Ancient Greece, it was conceived of as rule by the best qualified citizens, and contrasted with monarchy...

/plutocracy

Plutocracy

Plutocracy is rule by the wealthy, or power provided by wealth. The combination of both plutocracy and oligarchy is called plutarchy. The word plutocracy is derived from the Ancient Greek root ploutos, meaning wealth and kratos, meaning to rule or to govern.-Usage:The term plutocracy is generally...

 (the Magnate

Magnate

Magnate, from the Late Latin magnas, a great man, itself from Latin magnus 'great', designates a noble or other man in a high social position, by birth, wealth or other qualities...

s).

The specific acts of law that constituted the incompatibilitas rule were bans on holding:

• a district office simultaneously with a starosta
  Starosta
  Starost is a title for an official or unofficial position of leadership that has been used in various contexts through most of Slavic history. It can be translated as "elder"...
  
  's office (przywilej czerwiński, 1422);
• two separate district offices;
• a voivode's office simultaneously with a starosta's office (przywilej nieszawski, 1454);
• a kanclerz
  Kanclerz
  Kanclerz was one of the highest officials in the historic Poland. This office functioned from the early Polish kingdom of the 12th century until the end of the Polish-Lithuanian Commonwealth in 1795. A respective office also existed in the Grand Duchy of Lithuania since the 16th...
  
  's (chancellor's) office simultaneously with a starosta's, voivode's or castellan
  Castellan
  A castellan was the governor or captain of a castle. The word stems from the Latin Castellanus, derived from castellum "castle". Also known as a constable.-Duties:...
  
  's office;
• a voivode's or castellan's office simultaneously with a starosta's office (1538);
• two or more city starosta offices (starostwo grodowe) (1562 or 1563);
• a voivode's and castellan's offices simultaneously with another ministerial office other than that of hetman
  Hetman
  Hetman was the title of the second-highest military commander in 15th- to 18th-century Poland and the Grand Duchy of Lithuania, which together, from 1569 to 1795, comprised the Polish-Lithuanian Commonwealth, or Rzeczpospolita....
  
   (1565, as each of those offices, except that of hetman, granted its holder a seat in the Senate of Poland
  Senate of Poland
  The Senate is the upper house of the Polish parliament, the lower house being the 'Sejm'. The history of the Polish Senate is rich in tradition and stretches back over 500 years, it was one of the first constituent bodies of a bicameral parliament in Europe and existed without hiatus until the...
  
  , a Privy council
  Privy council
  A privy council is a body that advises the head of state of a nation, typically, but not always, in the context of a monarchic government. The word "privy" means "private" or "secret"; thus, a privy council was originally a committee of the monarch's closest advisors to give confidential advice on...
  
   rather than an upper chamber).http://www.bezuprzedzen.pl/urzedy/poczatki.html

Commonwealth ministers were the equivalents of modern central-government officials. They comprised 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. Lithuania

Lithuania

Lithuania , officially the Republic of Lithuania is a country in Northern Europe, the biggest of the three Baltic states. It is situated along the southeastern shore of the Baltic Sea, whereby to the west lie Sweden and Denmark...

n ministers, while enjoying the same powers as Crown

Crown of the Polish Kingdom

The Crown of the Kingdom of Poland , or simply the Crown , is the name for the unit of administrative division, the territories under direct administration of Polish nobility from middle-ages to late 18th century...

ministers, were considered to stand lower in the hierarchy. Hetmans were also considered "ministers," but had no seat in the Senat.