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The pro formation is the basic formation mostly used by any offense. It is defined by having the Tight End (or Y receiver) up on the line next to the offensive lineman. This however causes the offensive line to be unbalanced towards the tight end side (meaning they have more players on that side of the ball) hence it is ussually considered the "strenght" side of the ball.
A pro-left formation indicates that the tight End is on the left side of the ball.
A pro-right formation indicates the tight end is on the right-side of the ball.