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Bolesław was the eldest son of the Polish high duke Henry II the Pious by his wife Anna, a daughter of the Přemyslid king Ottokar I of Bohemia. His paternal grandparents were Henry the Bearded and Hedwig of Silesia. Among his younger siblings were Mieszko (died 1242), Henry III the White (d. 1266), Konrad II (d. 1274), Władysław, and Elisabeth, who married her Piast cousin Duke Przemysł I of Greater Poland.