Duchy of Czersk in the context of Siemowit III, Duke of Masovia

The Duchy of Masovia was a district principality and a fiefdom of the Kingdom of Poland, existing during the Middle Ages. The state was centered in Mazovia in the northeastern Kingdom of Poland, and during its existence, its capital was located in the Płock, Czersk and Warsaw. It was formed in 1138 from the territories of the Kingdom of Poland, following its fragmentation, that was started by the testament of Bolesław III Wrymouth. The country existed in the years: 1138–1275, 1294–1310, 1370–1381, and 1495–1526, between that time, going through fragmentations of its territory into smaller duchies and its unification. The states formed during its fragmentation were duchies of Kuyavia, Dobrzyń, Czersk, Płock, Warsaw, Rawa and Belz. In 1526, the country was incorporated into the Kingdom of Poland.

Czersk (Polish pronunciation: [t͡ʂɛrsk] ) is a settlement in the administrative district of Gmina Góra Kalwaria, within Piaseczno County, Masovian Voivodeship, in east-central Poland. The village also lies on the Czersk Lake (size: 9 hectares), which is an oxbow lake of the Vistula.

Czersk was an important settlement in the past and is one of the oldest Mazovian cities, famous for ruins of a medieval castle constructed in the late 14th century. It was a royal town of the Crown of the Kingdom of Poland in the past. In 1247–1526, it was the capital of the Duchy of Czersk, one of medieval Polish duchies. After incorporation of Mazovia into Poland (1526), the duchy was turned into the Czersk Land (see ziemia), part of Masovian Voivodeship (1526–1795). The Land of Czersk was divided into four counties - those of Czersk, Grójec, Garwolin (since 1539), and Warka. Czersk itself was the seat of castellans, who also were senators of the Polish–Lithuanian Commonwealth.

The Duchy of Warsaw was a feudal district duchy in Masovia, centered on the Warsaw Land. Its capital was Warsaw.

The state was established in 1310, in the partition of the Duchy of Masovia, with duke Siemowit II of Masovia becoming its first leader. It existed until 5 November 1370, when, under the rule of duke Siemowit III, duchies of Czersk, Rawa, and Warsaw were unified into the Duchy of Masovia. It was again re-established in June 1381, in the partition of the Duchy of Masovia, with duke Janusz I of Warsaw as its first leader. It existed until 1488, when it got incorporated into the Duchy of Czersk.

The Duchy of Rawa was a feudal district duchy in Masovia, centered on the Rawa Land. Its capital was Rawa. It existed during the High Middle Ages era, from 1313 to 1370, and from 1381 to 1442.

The state was established in April 1313, in the partition of the Duchy of Płock, with duke Siemowit II of Masovia becoming its first leader. It existed until 5 November 1370, when, under the rule of duke Siemowit III, duchies of Czersk, Rawa, and Warsaw were unified into the Duchy of Masovia. It was again reestablished in June 1381, in the partition of the Duchy of Masovia, with duke Siemowit IV as its first leader. It existed until 1488, when it got incorporated into the Duchy of Czersk.