126 was a cartridge-based photographic film format introduced by Kodak in 1963 for its range of Instamatic cameras. The number 126 had previously been used for an unrelated roll-film format.
The number 126 comes from the dimensions of the negatives, 26.5mm square.
It has a continuous paper backing, and the frame number is visible through a small window at the rear of the cartridge. Cameras for this type of film were equipped with a larger rectangular window in the back door, through which was visible not only the frame number, but also a portion of the label showing the film type and speed. The cartridge has a captive take-up spool, but no supply spool, with the film simply coiled tightly in the supply end of the cartridge.
The film is unperforated, except for one registration hole per image. A sensing pin in the camera falls into this hole when the film is fully advanced to the next frame, at which point the winding knob or lever is locked. The film does not need to be rewound, and is very simple to load and unload.
The film is pre-exposed with frame lines and numbers, a feature intended to make printing and viewing easier. The top edge of the cartridge above the film gate has a square notch in a specific position corresponding to the speed of the film in the cartridge, used by some higher-end cameras.
Originally available in 12 and 20 exposures, by the time regular production stopped it was only available in 24 exposure cartridges.
Around 10 million 126 film cameras were made by Kodak and other companies, mostly fairly simple amateur models. Kodak stopped making 126 Instamatic cameras in 1988, and stopped making 126 film in 1999, but some 126 film has been produced by other companies since.