The Leica was initially conceived as an exposure test device for 35 mm cinema. The 35 mm cine camera's gate is 24 x 18 mm. That's the so-called Edison format and yes, T. A. Edison invented it. By convention -- set to gain working distance -- the "normal" focal length for 35 mm movies is ~ 50 mm. That's what Oscar Barnack used on his exposure tester. When the exposure tester grew into a camera that should double frame 35 mm (24 x 36) the 50 mm lens was retained.