What it is

In an unbalanced line the offense puts more linemen on one side of the center than on the other. An extra tackle or a tight end usually slides over to join the heavy side.

In a game

The defense must decide whether to shift its own front. The heavy side has an extra blocker, so runs in that direction gain power, while a defender may be left unblocked on the light side.

More detail

Rules limit what the extra lineman can do: he must stay on the line and, if he wears an ineligible number, he cannot catch a pass. Because of this, the formation also works as a signal. Many teams use it to change who is eligible and to confuse the defense about which player will go out for a pass.

Animated diagram

1. The offense puts an extra tackle on the right, so there are three linemen to the right of the center and only two to the left.
2. The defense has to decide whether to move its own players over to that side.
3. The play goes to the strong side, where the extra blocker gives it more power.