The United States Golf Association (USGA) is the final authority on everything having to do with how the game is played, including having aegis over the equipment golfers can use. With regard to the shafts that are sanctioned by the USGA, there are three rules governing them. The rules set out what's allowed as to the straightness of shafts, their ability to bend or twist and how they should be attached to the club head.
Shaft Must Be Straight
The USGA rule reads: “The shaft of the club must be straight from the top of the grip to a point not more than 5 inches (127 mm) above the sole, measured from the point where the shaft ceases to be straight along the axis of the bent part of the shaft and/or socket.” This rule is self-explanatory except it implies the shaft should not extend beyond where the butt cap of the grip is normally placed. The tricky part of this rule is determining where the shaft ends and the club head begins, so the USGA has mandated it should not exceed 5 inches long from the top of the hosel (where the shaft is inserted) to the bottom or sole of the club head. This rule has special significance because shafts of putters can be attached at the end of the club head or in the middle of it.
“Bendability And Twistability"
The USGA rule about this reads: "At any point along its length, the shaft must bend in such a way that the deflection is the same regardless of how the shaft is rotated about its longitudinal axis; and twist the same amount in both directions." This rule prevents a manufacturer or the golfer from taking unfair advantage of the variances that are present in virtually all shafts by attaching them to club heads in such a way as to influence their performance. For example, say a golfer prefers a club flex to a particular degree on his downswing. A typical graphite shaft has so-called “spines,” so how it is attached to the hosel will affect how much it bends during the shot. The USGA, in the same rule, prevents using the twisting action of a shaft to a golfer's advantage.
How A Shaft Is Attached
The USGA rule covers this aspect as follows: "The shaft must be attached to the club head at the heel directly or through a single plain neck and/or socket. The length from the top of the neck and/or socket to the sole of the club must not exceed 5 inches (127 mm), measured along the axis of, and following any bend in, the neck and/or socket." The hosel must be plain and it cannot be used for any other purpose. And the club head cannot be longer than 5 inches. Some club heads are constructed to fit inside the shaft. In that case, you should begin measuring the club head from the end of the shaft instead of the top of the hosel. Finally, because putters come in a huge variety, there is a USGA rule that states the place where the club head is attached be fixed, not moveable.