The Duchy of Sieradz (Latin: ducatus Siradiae, Polish: Księstwo Sieradzkie), also known as the Duchy of Siradia, was created based on part of the Duchy of Łęczyca in 1263. Duchy of Łęczyca was created around 1231 from Province of Łęczyca, a part of the Seniorate Province by Konrad I of Masovia.
Ruled by the rivaling Masovian branch of the Piast dynasty. In 1299 Duke Władysław I the Elbow-high had to cede Sieradz to King Wenceslaus II of Bohemia, who had also obtained the Seniorate Duchy of Kraków in 1291. Nevertheless, upon the extinction of the Bohemian Přemyslid dynasty in 1306, it was reunited with the Kingdom of Poland as a vassal duchy, and after 1339 incorporated by King Casimir III the Great into the Lands of the Polish Crown as Sieradz Voivodship. Around that time, the term Sieradz Land (Terra Siradiensis, ziemia sieradzka) begun replacing the older Duchy nomenclature.
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