Precision mechanics: the buvette (pump room) of Évian-les-Bains is not just an architecture of great formal elegance, but above all an extraordinary technological object, a machine carefully calibrated, in equilibrium. Architectural criticism has largely dwelt on the quintessential nature of the building, a transparent parallelepiped, whose slender, airy, double-curved roof appears resting on the upper arms of steel béquilles. Behind an apparent simplicity, however, lies an ingenious and sophisticated structural and constructional conception, in which the characteristics of the materials are exploited to the limit of their resistance. But apart from the undeniable interest of the technological solutions adopted, what makes the Évian buvette a fundamental testament to the history of twentieth-century engineering is the genesis of the detailed design, perfected during the construction phase itself. The collaboration between the brilliant “mathematician of impossible forms” Serge Ketoff and the “blacksmith who became an architect and entrepreneur” Jean Prouvé, gave rise to a profoundly original work. After months of cross-calculations and close negotiations on the strength of the bolted timber/steel assemblages, the public certification bodies actually stated that the structure was simply “impossible to calculate” by conventional methods. Yet, apart from the theory of structures, put severely to the test, load tests of the first girder to be made revealed excellent stress resistance (attested today, moreover, by the building’s excellent state of preservation).