In American football and Canadian football, not all players on offense are entitled to receive a forward pass. Only an eligible pass receiver may legally catch a forward pass, and only an eligible receiver may advance beyond the neutral zone if a forward pass which crosses the neutral zone is thrown. If the pass is received by a non-eligible receiver, the penalty for the foul "illegal touching" is assessed (the play is treated as an incomplete pass, unless the ball is downed behind the line of scrimmage — in either case a down is lost). If an ineligible receiver is beyond the neutral zone when a forward pass which crosses the neutral zone is thrown, a foul of "ineligible receiver downfield" (penalty--a loss of yardage, but not loss of down) is called. Each league has slightly different rules regarding who is and is not considered an eligible receiver.
A receiver loses his eligibility by leaving the field of play unless he was forced out by a defensive player and immediately attempts to get back inbounds (Rule 7-3-4). All players on the field become eligible as soon as the ball is touched by a defensive player or an official during play (Rule 7-3-5).
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.
In many leagues eligible receivers must wear certain uniform numbers, so that the officials can more easily distinguish between eligible and ineligible receivers. In the NFL running backs must wear numbers 20 to 49, tight ends must wear numbers 80 to 89 (or 40 to 49 if the numbers 80 to 89 have been exhausted), and wide receivers must wear numbers 10 to 19 or 80 to 89. In the CFL ineligible receivers must wear numbers 50 to 69; all other numbers (including 0 and 00) may be worn by eligible receivers. A player who is not wearing a number that corresponds to an eligible receiver is ineligible even if he lines up in an eligible position. However, in the American game, a player who reports to the referee that he intends to be eligible in the following play is allowed to line up and act as an eligible receiver. An example of this was a 1985 NFL game in which William Perry, wearing number 72 and normally a defensive lineman, was made an eligible receiver on an offensive play, and successfully caught a touchdown pass attempt. A more recent example, and more commonly used, has been New England Patriots linebacker Mike Vrabel lining up as a tight end in goal line situations; he is currently a perfect 10-for-10 in such situations, with each catch being converted into a touchdown. In the Canadian game a player who wears an ineligible number may make himself an eligible receiver if the referee grants him permission to change his uniform to an eligible number at the team's bench.
Before the snap of the ball, in the American game, eligible receivers may only move parallel to the line of scrimmage, only one eligible receiver may be in motion at any given time, 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). However, in the Canadian game, eligible receivers may move in any direction before the snap, any number may be in motion at any one time, and there is no need to be motionless before the snap.
The rules on eligible receivers only apply to forward passes, even those behind the line of scrimmage. However, any player may legally catch a backwards or lateral pass.
In the American game, once the play has started, players can become ineligible and eligible depending on how the play develops. Any eligible receiver that goes out of bounds is no longer an eligible receiver and cannot receive a forward pass. Also, if a pass is touched by any eligible receiver (tipped by a defensive lineman, slips through a receiver's hands, etc.), every player on the field immediately becomes eligible. In the CFL all players become eligible receivers if a pass is touched by a member of the defensive team.