Hybrid designs often incorporated elements of the side-entry or three-piece designs. The Fibre Jet shared much in common with the Rosemount design, for instance. Side-entry Introduced by Rosemount in 1965, side-entry design consisted of an almost completely enclosed shell with a cut-out section on one side. The cut-out was covered by a flap that hinged along the back of the boot, swinging to the rear to open. Stepping in was very easy, simply sliding the foot sideways in through the opening, then swinging the flap closed and stretching a fabric cover over it to seal it. As the upper and lower sections both opened, metal plates were needed on the sides to connect the two mechanically. A problem was that the boot did not meet perfectly along the join, allowing snow to force its way into the boot, although improvements were continuous. This design fell from use in the 1970s as higher-cuff front-entry boots became largely universal. External frames The ski boot provides four functions; protecting the foot from the elements, providing a mounting point for the binding, and transmitting forces between the leg and the ski. In theory, there's no reason these have to be combined in a single unit, and several designs have split these functions up. One example is the Nava System from the 1980s, which used a soft boot that clipped into custom bindings, and an arm that extended up from the rear binding to wrap around the leg and provide lateral control. Knee-highs In 1980 four designs were introduced that all rose to a point just under the knee. They were normal ski boots below, but used an extended tongue that fastened around the upper leg using a variety of methods. They offered much greater edging control, and were quickly copied by many other companies. They all disappeared by 1983, a victim largely of fashion - ski pants wouldn't fit over them. None are produced today. Nordic Cross-country and telemark Cross-country boots, like all Nordic equipment, attach to the ski usually only at the toe of the boot and are allowed to flex at the ball of the foot similarly to a normal shoe or boot. Cross-country boots generally use one of four attachment systems; NNN (New Nordic Norm), 75mm Nordic Norm ("three-pin" binding, "75NN"), d-ring, or SNS (Salomon Nordic System). A four-pin binding system similar to 75NN used to be popular in the USSR.