House of Pomerania
The tenth century far-traveled Arab writer Al-Masudi – who had great interest in non-Muslim peoples, including the various Slavs of Eastern Europe – mentions a people which he calls "Kuhsabin", who were probably Kashubians. The oldest known unambiguous mention of "Kashubia" dates from 19 March 1238 – Pope Gregory IX wrote about Bogislaw I as "dux Cassubie" – the Duke of Kashubia. The old one dates from the 13th century (a seal of Barnim I from the House of Pomerania, Duke of Pomerania-Stettin). The Dukes of Pomerania hence used "Duke of (the) Kashubia(ns)" in their titles, passing it to the Swedish Crown who succeeded in Swedish Pomerania when the House of Pomerania became extinct.