Why does film come in multiples of 12 exposures?

[terence_spross1]

40 years ago, all you could buy in 35mm was 20 and 36 exposure lengths.

I believe that the 35mm magazine was designed (early 1930s) to hold 40 exp based on the thickness of film then, but the leader length and loss at the end of the film meant only 36 exp was usable when loaded in a typical (Leica, or Retina) camera. the tounge length was extended slightly since the introduction.

 

So the length of 36 exp has been always available since then.

 

Before the disposable cameras Konica was selling 35mm in 27 exposure lengths.

[terence_spross1]

When I first bought the Illford 72 exposure size, I didn't consider the fact it wouldn't fit on my processing reel. Not willing to cut the film in the middle, I considered tray processing but realized my arms weren't that long ;-). So after a long time I eventually cut the film after remembering where the most important potos on the roll were. Upset that I cut a good photo in half - I never used the other rolls - and I still have them (expeired be a couple decades.) All the other B&W film I've ever used was Kodak.