A set of iron-type golf clubs includes long, mid- and short irons with channel back configurations and a mass control insert. The mass of the insert is systematically varied through the set such that the mass distribution properties of the set may be systematically varied while retaining a continuous look and feel through the set. The mass of the insert is varied by altering the volume or the density of the insert through the set. Additional design parameters for the set may also be systematically varied through the set, such as groove type and depth, loft angle, cavity volume, hitting face roughness, and sole width. In one embodiment, the mass control insert comprises a dense insert and a lightweight cover. The density of the dense insert can be easily varied to change the mass distribution properties of the club head. One application of the mass control insert is to provide customization of the club head at the point of sale or distribution.