The outer shell of the ball used in NFL professional games or NCAA college games must be made of genuine leather and dyed into the coffee color of natural leather. Amateur entertainment balls are sometimes made of rubber or plastic. Modern leather making technology can press pebble patterns on the leather surface to increase friction, so that the ball will not slip out of the hand whether it is dry or wet. American footballs usually have thick white semicircular stripes (two pieces of leather on both sides of the seam) on both ends of the ball to increase the conspicuity in low light conditions, but the NFL ball does not have white stripes. Because early American footballs were made of inflated pig bladders, they are also commonly known as "pigskin".
According to official NFL regulations, the home team must prepare 36 leather balls for outdoor games and 24 leather balls for indoor games two hours before the start of the game. The referee uses a barometer to measure the ball to meet league regulations. Twelve new footballs, sealed in special boxes, are shipped by the manufacturer and opened by the referee in the locker room two hours before the start of the game. These footballs must be specially marked with the letter "k" and are dedicated to kicking games.
Field requirements
The American football field is often nicknamed the "Gridiron" because the vertical and horizontal markings on the field make the field look like a wire rack that can help food be grilled on the stove. For this reason, American football and the similar Canadian football are collectively referred to as "gridiron football". Because American football is more representative of the two, other English-speaking countries even simply refer to it as "gridiron" to refer specifically to American football.
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