The ranges are subdivided by several deeply indented river valleys, mostly running east-west, including the [[Inn (river)|Inn]], [[Salzach]], [[Enns (river)|Enns]], [[Adige]], [[Drava]] and [[Mur (river)|Mur]] valleys. According to the traditional [[Alpine Club classification of the Eastern Alps]] (AVE) widely used by Austrian and German [[Mountaineering|mountaineers]], these mountain chains comprise several dozen smaller mountain groups, each assigned to four larger regions: