Seriously - 35mm film is 35mm wide. The standard image measures 24mm x 36mm.
35 - 24 = 11mm left for side margins with sprocket holes.
That's it. Here are some more historical details.
- 35mm film started out as a movie film, with the whole film strip 35mm wide with 11mm used by sprocket holes and space for the sound track, leaving frames 24mm wide, and 18mm high. That gave the 4:3 shape of old movies, adopted successively for TV screens, computer monitors, video camera CCD's and most digital still camera sensors.
- When Leica adapted rolls of 35mm movie film for the higher resolution needs of still photography, they went for a larger frame by the mechanically simple strategy of doubling the length along the film, getting the now familiar 24x36mm. This used to be called "double frame" but is now mostly just called "35mm".
- Movies no longer need the soundtrack space, so now often use a "super 35mm format" with frame width about 25mm. Movie frame height has also shrunk to about 13mm, due to the shift to wider screen formats for movies.
Note: If you read about sensors for digital 35mm format movie cameras, the frame size is still roughly the traditional 24x18mm, so about the same frame width as Nikon, Fuji, Pentax, Konica-Minolta and Sony make and use in their DSLRs. So all these companies could argue that their DSLR's use roughly the original 35mm format!