Have you noticed how many people refer to 120 film as "120 mm film"?
Lately I’ve been noticing lots of people saying or writing “120 mm film” when they are discussing 120 film. 120 is a number assigned by Kodak to identify this film. The actual width of the film is 6 cm, or 60 mm. 120 mm is approximately 4 3/4 inches. Think of the 4 × 5 film used in view cameras; 120 mm film would be too wide to fit in the film holders for these view cameras if you cut a five inch strip.
There are different films that are identical in width, but have different numbers because of differences such as length, paper backing changes or spool dimensions. 620, for example, is the same as 120 except that it’s on a smaller spool. 220 is a double-length roll of 120 film, but lacks a paper backing except for the leader and trailer. There is also 105 and 117, older films that had different spools.
I have wondered if the 20 in 120 refers to the image width being two inches. Actually it’s a slightly wider than two inches. I read that 126 got its name because of 26 mm wide images (which were actually a little wider than 26 mm) which made me wonder about the name of 120. I’m just speculating on 120. 126 was a special 35 mm film loaded in a cartridge.
I’m just waiting for someone to start calling the film in the miniature 110 cartridge “110 mm film”.