Nassau, originally a county, developed on the lower Lahn river in what is known today as Rhineland-Palatinate. The town of Nassau was founded in 915. Dudo of Laurenburg held Nassau as a fiefdom as granted by the Bishopric of Worms. His son, Rupert, built the Nassau Castle there around 1125, declaring himself "Count of Nassau". This title was not officially acknowledged by the Bishop of Worms until 1159 under the rule of Rupert's son, Walram. By 1159, the County of Nassau effectively claimed rights of taxation, toll collection, and justice, at which point it can be considered to become a state. The Nassauers held the territory between the Taunus and the Westerwald at the lower and middle Lahn. By 1128, they acquired the bailiwick of the Bishopric of Worms, which had numerous rights in the area, and thus created a link between their heritage at the lower Lahn and their possessions near Siegen. In the middle of the 12th century, this relationship was strengthened by the acquisition of parts of the Hesse-Thüringen feudal kingdom, namely the Herborner Mark, the Kalenberger Zent and the Court of Heimau. Closely linked to this was the "Lordship of Westerwald", also in Nassau's possession at the time. At the end of the 12th century, the House acquired the ReichshofWiesbaden, an important base in the southwest. 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.
County of Nassau-Weilburg
Walram's son Adolf became King of Germany in 1292. His son Count Gerlach abdicated in 1344 and the County was divided under his sons in 1355
County of Nassau-Weilburg, again divided from 1442 to 1574
County of Nassau-Wiesbaden, again divided from 1480 to 1509
*County of Nassau-Idstein
*County of Nassau-Wiesbaden
fell back to Nassau-Weilburg in 1605
County of Nassau-Sonnenberg, partitioned among Nassau-Wiesbaden and Nassau-Weilburg in 1405
In 1605, all parts of Nassau-Weilburg were again unified under Count Louis II; however, after his death in 1627, his sons divided the county again
County of Nassau-Idstein, fell to Nassau-Ottweiler in 1721
County of Nassau-Saarbrücken, divided again in 1640
*County of Nassau-Saarbrücken, fell to Nassau-Ottweiler in 1723
*County of Nassau-Ottweiler, fell to Nassau-Usingen in 1728
*County of Nassau-Usingen, Principality in 1688
County of Nassau-Weilburg
After Nassau-Usingen had inherited Nassau-Ottweiler with former Nassau-Idstein and Nassau-Saarbrücken, it was reunified with Nassau-Weilburg and raised to the Duchy of Nassau in 1806.
County of Nassau-Dillenburg
After the death of Count Otto I, his country was divided between his sons in 1303:
County of Nassau-Dillenburg, fell to Nassau-Siegen in 1328
County of Nassau-Hadamar, fell to Nassau-Dillenburg in 1394
County of Nassau-Siegen, called Nassau-Dillenburg from 1328 on, again got divided from 1341 to 1561: