The Leica M mount was introduced in 1954 at that year's
Photokina show, with the
Leica M3 as its first camera. The 'M' stands for
Messsucher or
rangefinder in German. This new camera abandoned the
M39 lens mount in favour of a new bayonet mount. The bayonet mount allowed lenses to be changed more quickly and made the fitting more secure. Other innovations introduced by the M3 included a single window for the
viewfinder (for
composition) and the rangefinder (for
focusing). With a double-stroke
film advance lever (later models have a single-stroke lever). The M3 was a success and over 220,000 units were sold, by the time production ended in 1966. It remains the best-selling M mount camera ever made. The M3 uses
135 film (or 35 mm film), with the canister being loaded behind a detachable bottom plate. The M3 was followed by many other M mount cameras, released over 40 years, with many of the basic concepts remaining in these designs. With the introduction of the
Through-the-lens metering (TTL) in the
Leica M5 and the
digital Leica M8 being the most notable innovations since then. The lenses for the M mount were also introduced in 1954 and were based on the earlier M39 thread mount. Almost all M mount lenses are
Prime lenses. These lenses are divided by Leica based on their maximum
aperture number (also known as
f-number). They are distinguished by their names: File:Leica M3 mg 3673.jpg|M mount female part of the bayonet on a Leica M3 body. The mechanical sensor seen inside the top of the mount is the rangefinder coupling arm File:Leica M3 mg 3671.jpg|M mount male part of the bayonet on the Leica Elmar 90mm . File:Leica M3 mg 3685.jpg|The Leica M3
shutter speed dial, film advance lever,
shutter button and the frame counter. File:Leica M3 mg 3628.jpg|The Leica M3 ASA dial, purely decorative reminder of the
Film speed showing DIN and ASA comparison dial. File:Leica Camera.svg|Leica "red dot" logo. ==M Mount camera bodies==