The Nikon F camera, introduced in April 1959, was Nikon's first SLR camera. It was one of the most advanced cameras of its day. Although many of the concepts had already been introduced elsewhere, it was revolutionary in that it was the first to combine them all in one camera. It was produced until October 1973 and was replaced by the quite similar Nikon F2. Aspects of its design remain in all of Nikon's subsequent SLR cameras, through the current Nikon F6 film and Nikon D4 digital models (which still share its Nikon F-mount for lenses). The "F" in Nikon F was selected by Nippon Kogaku from the letter F in 'Reflex' because the R was too difficult to pronounce for Japanese speakers. That tradition was carried all the way through their top line of Nikon cameras until the introduction of the Nikon D1 (digital) cameras decades later.