The R&A, and the United States Golf Association (USGA). Regulations The Rules of Golf, jointly governed by the R&A and the USGA, state in Appendix III that the diameter of a "conforming" golf ball cannot be any smaller than , and the weight of the ball may not exceed . The ball must also have the basic properties of a spherically symmetrical ball, generally meaning that the ball itself must be spherical and must have a symmetrical arrangement of dimples on its surface. While the ball's dimples must be symmetrical, there is no limit to the number of dimples allowed on a golf ball. Additional rules direct players and manufacturers to other technical documents published by the R&A and USGA with additional restrictions, such as radius and depth of dimples, maximum launch speed from test apparatus (generally defining the coefficient of restitution) and maximum total distance when launched from the test equipment. In general, the governing bodies and their regulations seek to provide a relatively level playing field and maintain the traditional form of the game and its equipment, while not completely halting the use of new technology in equipment design. Until 1990, it was permissible to use balls of less than 1.68 inches in diameter in tournaments under the jurisdiction of the R&A, which differed in its ball specifications rules from those of the USGA. This ball was commonly called a "British" ball, while the golf ball approved by the USGA was simply the "American ball". The smaller diameter gave the player a distance advantage, especially in high winds, as the smaller ball created a similarly smaller "wake" behind it. Aerodynamics When a golf ball is hit, the impact, which lasts less than a millisecond, determines the ball's velocity, launch angle and spin rate, all of which influence its trajectory and its behavior when it hits the ground. A ball moving through air experiences two major aerodynamic forces, lift and drag. Dimpled balls fly farther than non-dimpled balls due to the combination of these two effects. First, the dimples on the surface of a golf ball cause the boundary layer on the upstream side of the ball to transition from laminar to turbulent. The turbulent boundary layer is able to remain attached to the surface of the ball much longer than a laminar boundary with fewer eddies and so creates a narrower low-pressure wake and hence less pressure drag.