ça s'est fait en deux temps je crois. Assez rapidement (dans les années 1960) en 2,39), et puis en 2,40 plus tard.
The initial SMPTE definition for anamorphic projection with an optical sound track down the side (PH22.106-1957), issued in December 1957, standardized the projector aperture at 0.839 × 0.715 inches (21.3 × 18.2 mm) (aspect ratio 1.17:1). The aspect ratio for this aperture, after a 2x unsqueeze, is 2.3468…:1 which rounded to the commonly used value 2.35:1. A new definition was issued in October 1970 (PH22.106-1971) which specified a slightly smaller vertical dimension of 0.700 in. for the projector aperture to help make splices less noticeable to film viewers. Four-perf anamorphic prints use more of the negative's available frame area than any other modern format which leaves little room for splices; as a consequence, a bright line would flash onscreen when a splice was projected and theater projectionists had been narrowing the vertical aperture to hide these flashes even before issuance of PH22.106-1971. This new projector aperture size, 0.838 × 0.700 inches (21.3 × 17.8 mm), aspect ratio 1.1971…:1, made for an un-squeezed ratio of 2.39:1 (and commonly referred to by the rounded value 2.40:1 or 2.4:1). The most recent revision, from August 1993 (SMPTE 195-1993), slightly altered the dimensions so as to standardize a common projection aperture width (0.825-inch, or 21.0 mm) for all formats, anamorphic (2.39:1) and flat (1.85:1). The projection aperture height was also reduced by 0.01" in this modern specification to 0.825 × 0.690 inches (21.0 × 17.5 mm), aspect ratio 1.1956…:1 (and commonly rounded to 1.20:1), to retain the un-squeezed ratio of 2.39:1. The camera's aperture remained the same (2.35:1 or 2.55:1 if before 1958), only the height of the "negative assembly" splices changed and, consequently, the height of the frame changed.