The Duchy of Czersk was a feudal district duchy in Masovia, centered on the Czersk Land. Its capital was Czersk. The country was established in 1275, in the partition of the Duchy of Masovia, with duke Konrad II becoming its ruler. After his death,the duchy was unified with the Duchy of Płock, forming the Duchy of Masovia, on 24 June 1294. The state was again reestablished in 1310, with Trojden I, as its first ruler. 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 once again reestablished in 1471, from the part of the territories of the duchies of Płock and Warsaw. In 1488, it incorporated the Duchy of Warsaw, into its territory. It existed until 1495, when, with the incorporatio