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A player is out of bounds when any part of his body touches anything on or outside a boundary line (other than another player or official) (Rule 4-2-1).
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If in the normal process of taking a step any part of the foot touches the sideline, that foot is out of bounds.
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If the on-field officials rule a runner not out of bounds and allow an advance, replay can always look at whether the runner should have been out of bounds prior to the advance.
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If the officials rule the runner out of bounds replay can only look at the position of the ball at that point in relation to the line to gain or the goal line. The one exception is if the runner is ruled out but crosses the opponent's goal line in the immediate continuing action after the ruling.