Sit-skis are used for para-Nordic skiing and para-alpine skiing. If the sit-ski has one ski, it is sometimes name a mono-ski. If a sit-ski has two skis, it is sometimes called a bi-ski. Skiers first used bi-skis in para-alpine skiing before using mono-skis. Mono-skis allow skiers to ski using the same way that people who stand ski down a hill.