[1] A player is determined ineligible based on his position at the time of the snap.
When the ball is snapped, the offense is required to have no more than eleven players on the field, out of whom only six are eligible.
In the NFL, ineligible receivers must have at least part of their body no more than 1 yard beyond the line of scrimmage when a pass is thrown, unless they are engaged in a continuous block that began prior to leaving the 1-yard zone.
In college football, the NCAA allows ineligible receivers a maximum of 3 yards.
[1][6] The NCAA allows for an exception on screen plays, where the ineligible player is allowed to cross the line of scrimmage to go out and block when the ball is caught behind the line of scrimmage.