Rules of Eligibility
Passing the football is one of the main ways of advancing the ball downfield in the game of football. The passing game of a football team is vital to the team's success, but you can't just pass it to anyone on the team. There are certain rules of eligibility that dictate which players are allowed to catch the ball and which players are not. The main purpose of these rules of eligibility is to limit the number of players able to catch the ball in order for the defense to have a fair chance to defend against the passing game.
Just like the neutral zone rules (where only certain players are allowed beyond the neutral zone), an ineligible receiver who receives a forward pass is docked with a penalty. This penalty is called 'illegal touching' and results in a five-yard loss and a loss of down for the offense. This penalty is universal across the NCAA, CFL, and NFL. Players on defense are always eligible receivers.
In the NCAA, the determining factor of whether a player is an eligible receiver is their jersey number and their position on the field when the ball is snapped. Players on offense whose jersey number is between 50 and 79 are always ineligible. Players on the offense whose number do not fall between 50 and 79 are eligible only if they meet certain criteria: the player is at the end of the group of players on the line of scrimmage (this includes wide receivers and tight ends), the player is behind the line of scrimmage by at least one yard (running backs), or the player receives a snap from behind the center (usually the quarterback).
There are some exceptions to the eligibility rules such as a player wearing an eligible jersey number at an ineligible position if a special teams play is occurring. Also, if a player changes positions during the game, they must also change jersey numbers to reflect that position. This is done to help the referees in easily spotting ineligible receivers and players in the wrong position in the case of an illegal shift.
In the NFL, the offensive team must have seven players lined up on the line of scrimmage, and only two of those players (at either end of the line of scrimmage) are considered an eligible receiver. All of the players in the backfield (up to four in the NFL and five in the CFL) are eligible. The quarterback is eligible unless he is lined up behind the center in the I-formation (sometimes called the T-formation), then the quarterback is not eligible.
Just like in the NCAA, eligible receivers must wear certain jersey numbers. Running backs must wear numbers between 20 and 49, tight ends must wear numbers between 80 and 89 and 40 and 49, and wide receivers must wear numbers between 10 and 19 and 80 and 89. To be an eligible receiver, a player must be wearing an eligible jersey number. There are no exceptions unless a player reports to a referee that they will be lining up in an eligible position with the intent to catch a forward pass.
In the NFL, players behind the line of scrimmage may only move parallel to the line of scrimmage. If any forward movement occurs, the player must stand still for a full second before the snap to be considered eligible. When eligible receivers step out of bounds, that player loses eligibility to catch a pass unless they take three steps in-bounds to regain eligibility.
The CFL eligible receiver rules are almost the same as the NFL eligible receiver rules with a few slight differences. All ineligible players must wear numbers between 50 and 69, all other numbers may be worn by eligible receivers. Also, players behind the line of scrimmage may move in any direction prior to the snap and do not have to be motionless when the ball is snapped.
Eligible receiver rules have been manipulated in multiple ways to give a team an edge over their opponent. In 1985, William "The Refrigerator" Perry famously caught a touchdown pass after being made an eligible receiver. Perry, normally a defensive lineman and wearing the jersey number 72, lined up in an eligible position after notifying the referees of the teams intent.
In a 2015 game between the Patriots and Ravens, the Patriots ran a trick play in which a slot receiver was actually playing the right tackle position while in the slot. The lineman who was standing in the right tackle position was considered an eligible receiver while the wide receiver in the slot was ineligible. This resulted in a big play from the Patriots, much to the Ravens dismay.