The basement nappe concept arose during the excavation of the Simplon tunnel in the Central Alps. Recent detailed structural and petrologic investigations in this classic area have confirmed the concept in general outline. A complicated structural history has been deduced, however, with two phases of major folding and a period of large-scale faulting after the phase of nappe emplacement. The earlier of the subsequent fold phases was of regional importance and caused widespread inversion of the original structural succession. During this phase, the basement rocks were in their most ductile state and suffered the main part of their internal deformation.