Wilson Staff CI9 Irons, Set of 8 (4-PW, GW) The award winning head shape incorporates elements designed for Tour players. Rear cavity exoskeleton positions more weight to the heel and toe of the iron for a higher MOI and more stability. Solid feel and power. Soft 431 stainless steel in conjunction with one piece soft response insert that extends across the entire perimeter of the cavity creates a satisfying sound and feel at impact, even on off center hits. Modified U grooves push the maximum limits of the new specifications required by the USGA and R & A . A strategically positioned higher CG delivers a higher spin rate and promotes the ability to work the ball more effectively.