MOI (Moment of Inertia)

Moment of inertia, almost always shortened to MOI, measures how much a clubhead resists twisting when you miss the center of the face. The higher the MOI, the less the head rotates on a toe or heel strike, and the less distance and accuracy you lose on those mishits. In plain terms, MOI is the engineering behind the word "forgiveness."

Manufacturers raise MOI by pushing weight to the perimeter and extremes of the clubhead - out to the heel and toe, back away from the face. This is why forgiving drivers look large and square and why game-improvement irons have thick perimeters and hollow cavities. The mass is positioned to fight twisting.

There is a tradeoff. Maximum MOI heads are larger and harder to shape shots with, and many better players accept lower MOI in exchange for the workability and compact look of a smaller head. Higher handicaps almost always benefit from more MOI; low handicaps choose how much to give up for control.

MOI applies to putters too, where it governs how much the face stays square on off-center putts. The trend toward large mallet putters is largely an MOI story.