View-Master is a format for viewing still colour photographs in stereo vision, and was first introduced in 1939 by Sawyers Inc.
Each View-Master reel contains seven pairs of stereoscopic colour film images mounted in a cardboard disc, which when viewed together in a View-Master viewer combine to give a 3D image. Early reels contained images of tourist attractions and were intended as an alternative to the scenic postcard. They were also used for US military training during WWII.
In 1952, a View-Master Personal Stereo Camera was introduced to allow people to make their own reels. This was discontinued ten years later.
In 1966, Sawyers was acquired by GAF and this saw a move away from scenic views towards reels aimed at children containing cartoons and images from television series. From 1970 to 1997, there were versions of Talking View-Masters which included audio as well as images.
Despite 25 different View-Master viewer models and over 1.5 billion reels sold, the basic design stayed the same and every reel will work in every viewer.