A football is an inflated ball used to play one of the various sports known as football. The first balls were made of natural materials, such as an inflated pig bladder, sometimes inside a leather cover, which has given rise to the United States slang-term "pigskin." Modern balls are designed by teams of engineers to exacting specifications, with rubber or plastic bladders, and often with plastic covers. Various leagues and games use different balls, though they all have one of the following basic shapes: The precise shape and construction of footballs is typically specified as part of the .