[BadBoyBenny]

I plagarized this from Wiki
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With the assignment of numbers to positions, a player who is not wearing a number that corresponds to an eligible receiver is not eligible even if he lines up in an eligible position. However, in the American game, a person who reports to the referee that he will be eligible on the play is allowed to line up and act as an eligible receiver.

If, for example, eight men line up on the line of scrimmage, the team loses an eligible receiver. This can often happen when a flanker or slot receiver, who is supposed to line up behind the line of scrimmage, instead lines up on the line of scrimmage between the offensive line and a split end. In most cases where a pass is caught by an ineligible receiver, it is usually because the quarterback was under pressure and threw it to an offensive lineman out of desperation.

Eligible receivers may only move parallel to the line of scrimmage, only one eligible receiver may be in motion at any given time before the snap of the ball, and if forward motion has occurred, the receiver must be still for a full second before the snap. The receiver may be in motion laterally or away from the line of scrimmage at the snap. A breach of this rule results in a penalty for illegal procedure (five yards).

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