Shaft Flex
Also called: shaft stiffness
How much a golf shaft bends during the swing - rated from Ladies (L) and Senior (A) through Regular (R), Stiff (S), and Extra Stiff (X).
Shaft flex describes how much the shaft bends under load as you swing. It is the single spec golfers obsess over most, and the one most often guessed wrong. The letter ratings - L, A, R, S, X - are not standardized across manufacturers, so a Stiff from one brand can play softer than a Regular from another.
Flex is driven primarily by how fast and how aggressively you load the club, not by your gender or your age. A golfer with a 95 mph driver swing and a smooth tempo may need a different flex than someone swinging 95 mph with a violent transition. Tempo and the point at which you apply force matter as much as raw speed.
Get it wrong and the misses are predictable. Too stiff for your swing and the ball tends to come out low and to the right (for a right-handed golfer) with a flat, dead feel. Too soft and you get a high, hooky ball flight and a shaft that feels like it is lagging behind your hands. The right flex feels like the club is loading and unloading in sync with your motion.
Flex is also club-dependent. Most golfers play one flex through the irons but should not assume the same letter carries over to the driver, where speed is highest, or to wedges, where a softer feel is often preferred.