Along the Grenoble‐Pelvoux‐Viso transect in the Western Alps, we schematize the geometrical relationships between (1) ductile, marly‐calcareous, sedimentary covers and (2) their more rigid substratum. This substratum may be the pre‐Alpine granitic‐gneissic basement, the Triassic dolomites, or even the Jurassic ophiolites. Basement mullions, a few kilometer size, are characteristic of the external part of the Western Alps arc (Dauphiné zone). They strongly contrast with the dolomitic or ophiolitic boudins, also a few kilometer size, in the inner part of the arc (Piedmont zone). The Dauphiné megamullions formed and then became accentuated, during the Oligocene and then the Neogene tectonic phases, which correspond to the last major Alpine contractions, at relatively shallow structural levels. In the sedimentary cover, upright or inclined folds were associated with slaty or strain‐slip cleavage development and low grade metamorphism. The corresponding stretching directions, locally subvertical, trend normal to the regional fold and mega‐mullions axes, and to the mountain belt itself. The Piedmont megaboudinage took place during the earlier, major Eocene, Alpine phase (which, but for some exeptions, did not affect the Dauphiné zone). In the very ductile “Schistes lustrés” that surround the dolomitic and ophiolitic megaboudins, generalized intrafolial recumbent folding was associated with blueschist metamorphism. The deformation, characteristic of a deep structural level, was a strong flattening within a subhorizontal plane and a preferential elongation parallel to the fold axes. Owing to regional horizontal festooning, those axes trend normal, oblique, or even parallel to the Piedmont zone, i.e., to the incipient mountain belt (restored initial trends). Coeval megaboudinage was probably omnidirectional (chocolate tablet pattern). The Eocene Piedmont structure has not been really obliterated during the later Alpine deformations leading to megamullions development in the Dauphiné zone. We summarize the very numerous stratigraphical, sedimentological, and tectonic field observations which lead us to regard each Dauphiné megamullion as directly inherited from a previous pre‐Alpine faulted block. Each Piedmont megaboudin has a similar origin. The corresponding paleofaults are in fact normal and/or strike‐slip faults of truly Tethyan age: Their synsedimentary offset occurred either during the Liassic‐Middle Jurassic passive margin rifting evolution of the southern edge of the European continent or during the Late Jurassic opening up of the Ligurian oceanic Tethys. We analyze how those Tethyan faults partly induced the style of the Alpine collisional structures, according to the regional characters of the successive synmetamorphic deformations, i.e., to their localization in t