The circumference of the ball must not be more than 2.25 feet (0.69 m), and the ball cannot weigh more than 16 pounds (7.26 kg). The ball must have a smooth surface over its entire circumference except for holes or indentations used for gripping the ball, holes or indentations made to bring the ball back into compliance with weight-distribution regulations, identification letters and numbers, and general wear from normal use. For much of the history of bowling, bowling balls were made using a three piece construction method. Starting in the mid 1990s, however, most manufacturers switched to a two-piece method. In response to these innovative ball designs, the United States Bowling Congress placed further restrictions on the technical characteristics of the ball such as the radius of gyration and hooking potential.