For iron clubs, the “impact area” is deemed to be that part of the face which lies within 0.79 inches (20 mm) either side of the vertical centre line of the face, but excluding strips 0.25 inches (6.25 mm) wide from the top and bottom edges.
For metal wood clubs, the shape of the impact area is generally based on the traditional insets used in persimmon type wood clubs. Figure 7 illustrates the shape and dimensions of this area.
The height (h) of the trapezium is used to fix the horizontal dimensions namely “1/2h” at the top and “h” at the base. This ensures that all impact areas are similar.