Racquetball is a racquet sport playe with a hollow rubber ball in an indoor or outdoor court. Earl Riskey developed paddleball in the 1920's. Joe Sobek is credited with inventing the sport afterward in 1950, adding a stringed racquet to the game to increase velocity and control. Unlike most racquet sports, such as tennis or badminton, the court's walls, floor, and ceiling are legal playing surfaces, rather than out-of-bounds.