In the Third phase of Eventing, Show Jumping, courses are designed to test the horses' and riders' ability to negotiate a variety of fences of differing heights, widths, and technicality. This requires the horse to be balanced and supple for tight turns and short distances between fences. He must be able to lengthen and/or shorten his stride in an instant. Therefore, the rider must know exactly where he is on the approach to a fence, and have an obedient horse that will respond to his commands. For the spectator, this sport is both exciting and breathtaking to watch, as just one single rail knocked down can change the standings dramatically.