Frequency matching tunes a set of shafts so their stiffness — measured in cycles per minute (CPM) on a frequency analyzer — steps up evenly from long irons to short irons rather than jumping around. A builder clamps the butt end of the shaft at a fixed point (commonly 5 or 7 inches from the grip), pulls the tip down 3–5 inches, releases it, and counts the oscillation rate, averaging three readings per club. Two rules of thumb guide the work: roughly 4 CPM of increase per half-inch of length lost through an iron set, and about 10–12 CPM between adjacent flexes of the same shaft model, so a shaft reading 300 CPM plays a full flex softer than one at 312 CPM. Frequency matching catches the outlier club in a set — one that measures off the progression despite carrying the same flex label — before it reaches a member's bag.