The box is the area on the defensive side of the ball close to the line of scrimmage directly across from the offensive line.
The box is usually about as wide as a tight end would be and about as deep as five yards. The number of defenders in the box is typically an indicator of how committed the defense is to stopping the run on that play. Many offenses count the numbers in the box and make calls based on whether those numbers are favorable for running or not.
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