and, within the Rhine system, the watershed between the rivers Lahn and Sieg.

The source is at an elevation of 600 m and is located southwest of the 624 m high Lahnkopf. In the vicinity are also the origins of the Eder (5.5 km northwest of the Lahnhof) and the Sieg (another 3 km north). Whereas the Sieg takes the shortest route to the Rhine (to the west), the Lahn first runs in the opposite direction, paralleling the Eder for many kilometers.