Refers to Medium Format film, which comes in two standard lengths, 120 and 220, with 120 being the most common. The 220 size is simply twice as long as 120. Larger than 135 film, it is 61mm wide and does not have perforated edges (the film moves from one spool to another in-camera when shooting - no need to rewind after finishing a roll). 120 film generates anywhere from 10 - 16 exposures per roll, depending on the camera used. Typical sizes/ratios are 6x7cm (10 exposures), 6x6cm (12 exposures), and 6x4.5cm (16 exposures). 220 film does not have a protective backing paper on it (allowing the longer film to fit in the same spool as 120 film).