Climbing skins - often shortened to skins, and called feller in Norwegian - are the long adhesive strips of mohair, nylon or a mohair/nylon mix that fix to the base of an alpine touring ski for the climb. The fibres lie in one direction: they glide forward on the kick stride and grip the snow when weight is loaded backward, which is what allows a skier on topptur equipment to walk up a 30-degree slope without sliding back. At the top of the climb the skins peel off, fold sticky-side to sticky-side, and live in the pack until the next climb.
The name is historical. The original skins, used in the Alps and in Norway from the late nineteenth century, were strips of seal fur - hence seal skins - cut to the length of the ski and tacked to the wood. Modern skins are synthetic, with a glue layer that adheres directly to the ski base and a tip-and-tail attachment system that takes the structural load. The grip-to-glide compromise is real: pure mohair glides faster but wears out quickly, pure nylon grips harder but is slow on the kick stride. Most modern ski-tourers use a 70/30 mohair/nylon mix as the right compromise for serious distance days.
For the international visitor on a topptur trip in Norway, skins are part of the equipment that the lodge or guide provides as standard. The technique is straightforward and the lesson on the first morning takes about ten minutes. The cultural reference - skinning up a Norwegian peak before the descent - is the defining motion of the sport.