After the death of their brother Frederic the Fair on 13 January 1330 the Habsburg dukes Albrecht II and Otto the Merry shared the rule of their dominions for nine years. Otto, the youngest brother, had already claimed his share when Frederic had still been alive, which is why the only quarrel between the brothers worth mentioning had happened between 1327 and 1329. The joint rule lasted until Otto?s death in February 1339 and was characterised by brotherly concord and a prospering Habsburg dominion. By examining the charters issued by Albrecht II and Otto, this thesis shows how the two brothers managed their mutual rule. It becomes evident that Otto?s importance has been largely underestimated by most researchers. The young duke was mainly responsible for matters of foreign policy and was a great support to Albrecht II, who suffered from polyarthritis since 1330. The successful acquisition of the Duchy of Carinthia in 1335 is the best example of efficient brotherly cooperation. Typically, Otto was responsible for matters which required a "more mobile" duke, while Albrecht II ruled from Vienna. After Albrecht II returned from his pilgrimage in 1337, however, he became increasingly active in the joint rule. The analysis of the charters shows that whenever the dukes stayed in the same place, they used to issue charters together. When the brothers were separated, they generally drew up charters on their own. However, it seems that some legal acts, most notably service contracts, alliances, matters of foreign policy as well as longer-term property and mortgage transactions, required the consent of both brothers. This is why sometimes, one duke would issue a charter on his own as well as on his absent brother?s behalf.