Casimir III the Great
But on September 18, 1351, Siemowit and Casimir recognised the Polish King Casimir III the Great as suzerain, canceling the allegiance of Masovia and Bohemia. In exchange, they got Gostynin (Siemowit) and Sochaczew (Casimir), former possessions of Boleslaw III. Casimir also gave the Duchy of Płock to the brothers.