The M-mount is a lens mount, the Leica, the company (or its legal predecessor Leitz ) introduced in 1954 for rangefinder cameras.
It has a transfer function of the set on the lens distance to the rangefinder in the camera to use the measurement function of the measuring viewfinder can. The flange focal distance of 27.8 mm.
In the latest version, which has been introduced for the Leica M8, markers are also attached to the flange, which can provide an optically encoded information on the type of lens to the camera to allow the camera a digital image correction. Older lenses can be provided by Leica subsequently with the markings.
In addition to Leica offer or offered other companies cameras with this mount, so Konica Minolta (CL, CLE), Cosina ( Voigtlander ), Rollei, and Carl Zeiss.
These manufacturers have also offered each lenses with the M-mount, so the selection is appropriate lenses to one of the largest on the market. Other manufacturers were, inter alia, Avenon.
In camera systems with even lower flange and an equal or smaller image circle, such as the Micro Four Thirds System lenses can be used with M bayonet with a suitable lens adapter.