[grantsrc]

Knowing Theisey, I don't think he meant that a tackle can never be eligible. I think what he was getting at is that by definition, an uncovered player next to the guard would be an end, not a tackle. Even though he is the tradition tackle spot, he is not a "tackle".

Just to summarize what has been said here, no announcing, NFL only (one of my biggest pet peeves). In order to have a legal formation, you must have 5 guys on the LOS 50-79, so if you have a player with an eligible number 1-49 and 80-99, you must have 5 guys numbered accordingly. And like any other eligible receiver, he cannot be covered up by another player. This is for regular scrimmage downs.

For scrimmage kicks, the eligible receivers are determined when they set, not when the ball is snapped like a regular scrimmage down. You see this often confused on the swinging gate plays. The numbering exception comes into play after they shift to a kick formation, but if you move an eligible number to an eligible position, he must have been eligible in the original set, i.e. not covered by anyone prior to the shift.