Downhill biking (DH) is a gravity-assisted time trial mountain biking event. Riders race against the clock, usually starting at intervals of 30 seconds (seeded from slowest to fastest), on courses which typically take two to five minutes to complete. Riders are timed with equipment similar to that used in Downhill skiing. The placing is determined by the fastest times to complete the course; races are often won by margins of under a second. As the name of this discipline implies, downhill races are held on steep, downhill terrain with no extended climbing sections, resulting in high speed descents with extended air time off jumps and other obstacles.