Originally Posted by
GrumpyDiver
That's an easy one to answer, Brian.
In the beginning of modern film photography, sheet film was the common and cameras used 4" x 5", 5" x 7", 8" x 10", even 11" x 14" sheet film. These exist even today (vew cameras and field cameras). These were referred to as large format cameras. Even the Speed Graphic cameras that were in use by newspaper photographers in the 1940s through into the 1960's were sheet film cameras. These were collectively known as "large format cameras".
Along comes Oskar Barnack of Ernst Leitz GMBH and he builds a camera based on a double frame of motion picture film and we start down the historical path of the "small format camera" that ultimately ends up as our full-frame DSLR.
In the middle comes another innovation; a series of cameras based on roll film; the twin lens reflex Rolleiflex, Hasseblad Bronica, some of the Mamiya line, etc using a 2-1/4" x 2-1/4" (5cm x 5cm) and slight variations (6cm x 7cm) roll film (called 120 or 220 size (the difference being that 220 rolls were twice as long as 120 rolls). These were collectively known as medium format.