In 1255, after the Counts of Nassau acquired the estates of Weilburg, the sons of Count Henry II divided Nassau for the first time. Walram II received the county of Nassau-Weilburg. From 1328 on, his younger brother, Otto I, held the estates north of the Lahn river, namely the County of Nassau-Siegen and Nassau-Dillenburg. The boundary line was essentially the Lahn, with Otto receiving the northern part of the county with the cities of Siegen, Dillenburg, Herborn and Haiger and Walram retaining the section south of the river, including the cities of Weilburg and Idstein.