M39 is a screw mount system originally designed by Oskar Barnack for Leica cameras in the 1930s. These Leica Thread Mount (LTM) camera bodies were rangefinder cameras designed to accept interchangeable lenses by using a 39mm threaded mount. Many of the Leica cameras built with this mount featured separate rangefinders and viewfinders. Despite being designed by Leica, after World War II the M39 mount was used widely by companies like Canon, Nicca, Leotax, Zorki, and FED. These lenses can be easily adapted to Leica M and most modern mirrorless systems.