What it is

Receiver numbering is the way defensive coaches label the eligible receivers on one side of the formation. The receiver closest to the sideline is #1, the next one in is #2, and the third one in is #3.

In a game

The count always starts at the sideline and moves toward the ball, and it is done separately for each side. A wide receiver is usually #1, a slot receiver or a tight end is usually #2, and a tight end, a second slot receiver or a running back who lines up wide can be #3. A running back who stays behind the quarterback is not counted until he lines up out wide or runs out to the side.

More detail

The numbers matter because coverage rules are written with them: the cornerback takes #1, the apex takes #2, and the hook takes #3. A defender does not care who the player is, only where he stands in the count, so the same rule works against any formation. When a receiver goes in motion, the count changes and the defense has to count again.

The diagram

1. Each side of the formation is counted on its own.
2. The receiver closest to the sideline is #1.
3. The next receiver in is #2, and the one after him is #3.