The minimum allowed diameter of a golf ball is 42.67 mm and its mass may not exceed 45.93 g. Modern golf balls have a two-, three-, or four-layer design constructed from various synthetic materials. The surface usually has a pattern of 300-450 dimples designed to improve the ball's aerodynamics by reducing the ball's drag-inducing wake and allowing spin on the ball to create lift. The method of construction and materials used greatly affect the ball's playing characteristics such as distance, trajectory, spin and feel. Harder materials, such as Surlyn, usually result in the ball's traveling longer distances, while softer covers, such as Urethane, tend to generate higher spin, more "feel" and greater stopping potential. Golf balls are separated into three groups depending on their construction: two-, three-, or four-piece covers. Generally four-piece golf balls tend to be the most expensive, though price is no assurance of quality.