of Rheinfelden in 1090 (though not his comital title, which stayed with the family von Wetter-Rheinfelden). Berthold II is so named both as Duke of Swabia (following Berthold of Rheinfelden, the first duke of Swabia of this name) and as head of the House of Zähringen (following his father, who is counted as Berthold I of Zähringen in spite of not historically having used the name Zähringen). Berthold II did use the name Zähringen, although he moved his main residence from Zähringen Castle to the newly built Freiburg Castle in 1091. In 1092, Berthold II was elected Duke of Swabia against Frederick I of Hohenstaufen. In 1098, he reconciled with Frederick, renounced all claims to Swabia and instead concentrated on his possessions in the Breisgau region, assuming the title of Duke of Zähringen. He was succeeded in turn by his sons, Berthold III (d. 1122) and Conrad (d. 1152). In 1127, upon the assassination of his nephew Count William III, Conrad claimed the inheritance of the County of Burgundy against Count Renaud III of Mâcon. Renaud prevailed, although he had to cede large parts of the eastern Transjuranian lands to Conrad, who thereupon was appointed by Emperor Lothair III as a 'rector' of the Imperial Kingdom of Burgundy-Arles. This office was confirmed in 1152 and held by the Zähringer dukes until 1218. As a result, they are sometimes referred to as 'Dukes of Burgundy', although the existing Duchy of Burgundy was not an Imperial fief but a French one. Duke Berthold IV (d. 1186), who followed his father Conrad and founded the Swiss city of Fryburg (today's Fribourg-Freiburg) in 1157, spent much of his time in Italy in the train of Emperor Frederick I Barbarossa. His son and successor, Berthold V, showed his prowess by reducing the Burgundian nobles to order. This latter duke was the founder of the city of Bern in 1191, and when he died in February 1218, the ducal line of the Zähringer became extinct. Among other titles, the Zähringen family acted as Reichsvogt of the Zürichgau area. After the extinction of the ducal line in 1218, much of its extensive territory in the Breisgau and modern-day Switzerland returned to the crown, except for the allodial titles, which were divided between the counts of Urach (who subsequently called themselves the counts of Freiburg) and the counts of Kyburg, both descended from the sisters of Berthold V.