Flyball is run in teams of four dogs, as a relay. The course consists of four hurdles placed 10 feet (3 m) apart from each other, with the starting line six feet (1.8 m) from the first hurdle, and the flyball box 15 feet (4.5 m) after the last one, making for a 51-foot (15.5 m) length. The hurdle height is determined by the shoulder height of the smallest dog in the team. Under current North American Flyball Association (NAFA) rules this should be 5 inches (12.7 cm) below the withers height of the smallest dog, to a height of no less than 7 inches (20.3 cm) and no greater than 14 inches (40.6 cm). Each dog must return its ball all the way across the start line before the next dog crosses. Ideal running is nose-to-nose at the start line. The first team to have all four dogs cross the finish line error free wins the heat. Penalties are applied to teams if the ball is dropped or if the next relay dog is released early. Flyball is a dog sport in which teams of dogs race against each other from a start/finish line, over a line of hurdles, to a box that releases a tennis ball to be caught when the dog presses the spring-loaded pad, then back to their handlers while carrying the ball.