Skis designed for classic technique, both in track and in virgin snow, rely on a traction zone, called the "grip zone" or "kick zone", underfoot. This comes either from a) "texture", such as "fish scales" or mohair skins, designed to slide forward but not backwards, that is built into the grip zone of waxless skis, or from applied devices, e.g. climbing skins, or b) from "grip waxes". Grip waxes are classified according to their hardness: harder waxes are for colder and newer snow. An incorrect choice of grip wax for the snow conditions encountered may cause ski slippage (wax too hard for the conditions) or snow sticking to the grip zone (wax too soft for the conditions). Grip waxes generate grip by interacting with snow crystals, which vary with temperature, age and compaction. Hard grip waxes don't work well for snow which has metamorphosed to having coarse grains, whether icy or wet. In these conditions, skiers opt for a stickier substance, called "klister".