Progressive Wall Reduction System
Callaway Golf engineers lowered the center of gravity (CG) an additional 6 percent, progressively, throughout the X-20 set (over the X-18 Irons) for improved feel. 
Extreme Notch Weighting and 360-Degree Undercut Channel
Extreme Notch Weighting redistributes more weight to the extreme perimeter of the clubhead, creating a high moment of inertia (MOI) for greater forgiveness and more stability. The 360-Degree Undercut Channel maximizes perimeter weighting by moving the CG lower and farther back in the clubhead, enlarging the hitting area and stabilizing the clubhead for more forgiveness.VFT Technology maximizes ball speed and perimeter weighting for more distance and forgiveness. It allows Callaway Golf engineers to raise or lower the CG for optimum trajectory with each iron in the set, giving a higher launch angle in the long irons and better trajectory height in the short irons and wedges.S2H2 and Tru-Bore
The Short Straight Hollow Hosel (S2H2) increases discretionary weight by removing it from the hosel and repositioning it in the perimeter of the clubhead. The Tru-Bore design improves feel and clubhead control by allowing the shaft to extend through the hosel to the sole.

Variable Face Thickness (VFT) Technology