The difficulty levels of cross-country ski trails are generally assigned according to the distance and the relief, especially the descents. The inclination of the slopes is expressed as a percentage, a slope of 10% corresponding to an elevation gain of 32.8 ft (10 m) over a horizontal distance of 328 ft (100 m). Green trails, the easiest, have slopes of 0 to 13 %. Blue trails, intermediate, extend from 14 to 22%, and black trails, more difficult, from 23 to 30 %. The double black marking corresponds to extreme slopes of 30% inclination and more.