Nikon's first photographic system. Consisted of fully mechanical 35mm rangefinder cameras with focal plane shutter. The first camera of the system - Nikon I, had a 32x24mm frame format, models M and S increased it to a 34x24mm, and starting with the S2 it became the standard 36x24mm.
The cameras had a built-in lens helical and inner bayonet for 50mm lenses, and an outer bayonet mount for all other lenses. This design allowed much smaller 50mm lenses compared to other rangefinder mounts.
While the exterior mounts and back focus for Nikon and Contax rangefinder lenses were the same, the helical travel was different between the two mounts.
Nikon made a wide range of high-speed lenses with focal lengths from 21 to 1000mm for this system. Telephoto lenses with focal lengths starting from 180mm were designed for use with reflex housing only.
The code letter after the "Nikkor" engraving is indicative of the number of elements in each lens. The letters are from Latin or Greek: U for 1 element (Uns),B for 2 elements (Bini), T for 3 elements (Tres), Q for 4 elements (Quatuor), P for 5 elements (Pente), H for 6 elements (Hex), S for 7 elements (Septem), O for 8 elements (Octo), N for 9 elements (Novem), D for 10 elements (Decem).
Thus, the Nikkor-O 21mm lens was constructed with eight lens elements, and the Nikkor-T 105mm consisted of three elements.