[JJ_SO]

And the reason for most, if not all, sizes from film era, are more or less coincidentally. There were hardly artists saying "Oh, for my next landscape project I feel like I need 72 × 56 negative format, please make me one". It was more defined by the machines producing and cutting the films. As the 135 films always lost more than 30% of possible area due to perforation, that relation was way better with 120 rolls or LF single sheets. 

 

However, in film era there were few cameras which could produce various sizes by opening or closing the space like in a movie theatre opening the curtain - in digital era, various sizes are always at loss.