Downhill biking (DH) is a time trial mountain biking event held on a course with a net decrease in elevation. As the name of this discipline implies, downhill races are held on steep, downhill terrain, resulting in high speed descents, and with extended air time off jumps and other obstacles. A continuous course is defined on each side by a strip of tape. The width of the course can vary greatly over the length of the course, but it is typically between about 2m and 10m wide. Riders have one attempt to reach the finish line in the shortest amount of time while remaining between the tape. The rider must choose their path (or line) by compromising between the shortest possible line and the line that can be travelled a the highest speed. If a rider leaves the course by crossing or breaking the tape, she must return to the course at the point of exit. Riders start at intervals, often seeded from slowest to fastest. Courses typically take two to five minutes to complete and winning margins are often less than a second. Riders are timed with equipment similar to that used in Downhill skiing.