The NFL Rules Digest readily offers rules on how many balls must be provided at each game, but does not specify what makes a ball legal. This rule may be a reaction to the practice of some teams storing their own game balls at a different temperature than those to be used by their competitor.
The home club shall have 36 balls for outdoor games and 24 for indoor games available for testing with a pressure gauge by the referee two hours prior to the starting time of the game to meet with League requirements. Twelve (12) new footballs, sealed in a special box and shipped by the manufacturer, will be opened in the officials’ locker room two hours prior to the starting time of the game. These balls are to be specially marked with the letter “k” and used exclusively for the kicking game.
The NFL Rules Digest – www.nfl.com/rulebook/digestofrules
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