The flex in a golf shaft affects the distance, accuracy and trajectory of your golf shot. Because the shaft flexes during a swing, the position of the clubhead changes and it is important to select the proper flex in a club so that the clubface will be square on the ball when it makes contact. There are five flex ratings for golf clubs--extra stiff (X), stiff (S), regular (R), senior (A) and ladies (L). Generally, the faster your club speed, the stiffer the shaft, but use care when selecting a flex. If you are not certain or in between flexes, it is better to select a softer flex than a stiffer one. In addition, if you slice, a softer flex may help to correct the problem.
Extra stiff shafts should generally be reserved for really long hitters (about 300 off the tee) and this flex can be difficult to find, as it is not popular. It is important to note that if you select a shaft that is too stiff, you will lose loft and control on your shots.
The stiff shaft is quite popular and should be chosen by those who consistently drive the ball 250 or more yards off the tee. This shaft generally appropriate for mid-low handicaps, but be sure to have your clubhead speed measured before choosing between a stiff and regular flex. The stiff shaft may also help those with jerky swings.
High-handicap male golfers should generally choose regular flex clubs. This flex is appropriate for those who consistently drive the ball 230 to 250 yards off the tee. This flex is the most popular, as it is forgiving.
As men age, the speed of their golf swing slows down and if you reach the point where your drives are consistently between 200 to 230 yards, it is time to switch to the senior flex.
Women generally have the lowest club speed of all golfers and should select a ladies flex. This is the softest of all the flex ratings. If you consistently hit the ball less than 200 yards off the tee, this is an appropriate flex. Women who hit further may want to consider either senior or regular flex clubs.