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 ulna's length or the smallest dog's shoulder height on the team (depending on the association. For example, 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 (17.8 cm) and no greater than 14 inches (35.6 cm).