The loft angle of metal woods is designed into the club by creating a pitch to the face in relation to the sole. Then a number is marked on the sole of the club to indicate the loft of that particular club.
Playing loft, however, is the actual loft of the face pitch to the shaft plane. When the shaft plane is parallel to the face plane the playing loft is the same as marked on club. But when the face plane is either open or closed to the shaft plane the playing loft will vary from the loft marked on the sole.
The illustration shows the relationship of the face pitch to the shaft plane. As the shaft plane becomes closer to the face pitch plane the loft decreases (open club face). When the shaft plane is forward of the face pitch plane the loft increases (closed face).
The effect of open and closed face planes mostly relates to the spin imparted on the ball due to the increased or decreased angle between the shaft plane and face pitch plane.