Appendix II, 2a provides that:
The shaft 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 implies that the shaft must extend to the end of the grip, or at least that the grip should not extend beyond the top end of the shaft more than is necessary to accommodate the butt cap (see Figure 10).
The "5-inch" measurement should be made using a pair of calipers, to measure the depth of the head at the point where the shaft is attached, and a flexible measuring tape, or a piece of string, to measure along the axis of the shaft (see Figure 11).It is not possible to measure a bend in a shaft accurately using a normal, stiff ruler. The point where the shaft ceases to be straight can be determined by placing a rigid steel ruler along the straight part of the shaft and identifying the point where the shaft and the ruler are no longer in contact. This rule is particularly relevant for putters where the shaft is inserted directly into the head. (For putter heads with "necks," see Design of Clubs, Section 2c.)
b. Bending and Twisting Properties
Appendix II, 2b requires that:
At any point along its length, the shaft must:
(i) bend in such a way that the deflection is the same regardless of how the shaft is rotated about its longitudinal axis; and
(ii) twist the same amount in both directions.
This Rule effectively restricts shafts from being designed to have asymmetric properties, so that however the club is assembled, or whichever way the shaft is oriented, it will make no difference to the performance of the club.
However, many graphite shafts have a small "spine" running along the length of the shaft which may make them bend differently depending on how they are fitted to the head. As previously noted, the USGA recognizes that it is difficult for manufacturers to consistently produce a perfectly symmetrical shaft and, provided that the shaft is manufactured with the intention of meeting the above requirements, the USGA may incorporate a reasonable tolerance when evaluating shafts for conformance. Additionally, manufacturers of clubs may orient or align shafts which have spines for uniformity in assembling sets, or in an effort to make the shafts perform as if they were perfectly symmetrical. However, a shaft which has been oriented for the purpose of influencing the performance of a club, e.g., to correct wayward shots, would be contrary to the intent of this Rule.
It's difficult to assess the conformance of a shaft in the field. A standard shaft with a circular cross-section would most likely conform to the Rules unless there is evidence to the contrary (e.g., claims by the manufacturer which would indicate non-conformance, including advertising claims). However, a shaft which is not symmetrical in cross-section, such as an oval or rectangular shaft, would not normally be expected to conform to the Rules. Manufacturers of shafts with unique cross-sections should submit them to the USGA early in the development process, prior to production and marketing.
c. Attachment to Clubhead
Appendix II, 2c requires that:
The shaft must be attached to the clubhead at the heel either 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.
Exception for Putters: The shaft or neck or socket of a putter may be fixed at any point in the head.
The most important points to remember here are that a club must only have one neck, that it must be "plain", and, in order to restrict elaborate shapes and curves, the length of the neck is limited to 5 inches (127 mm). This measurement should be made in the same way as a bend in the bottom of a shaft (see Design of Clubs, Section 2a).
The majority of necks are designed to have the shaft inserted into them, and this normally avoids any confusion as to where the neck begins. However, if the neck is inserted into the shaft, the measurement should be taken from the end of the shaft.
The interpretation of a "plain" neck has recently been clarified as follows:
The neck must not be shaped for any purpose, other than connecting the shaft to the head in a traditional manner. While a neck may contain features such as an adjustability mechanism, a method for damping vibration or an alignment line, it must not be unusually shaped in order to house or accommodate such a feature. For example, in most cases, lines which have been painted or lightly engraved onto an otherwise plain neck are permitted. However, a neck designed specifically to accommodate such lines or marks would be considered non-conforming. Small scale features, on an otherwise plain neck, which are purely for decorative purposes, and could not effectively perform, or be used for, another purpose, may also be permitted.
The above requirements also apply to woods. However, there is some accommodation for the transition area between the head of a wood and its neck. This transition area must fit within a cylinder of a diameter and height of 1 inch (25.4 mm) measured from the base of the transition and parallel to the axis of the shaft. Any transition which satisfies this restriction should be permitted provided it does not contain any other non-plain feature (for example, holes or alignment bars).
NOTE: Some exceptions may be made for clubheads made of wood. Ferrules shaped to circumvent this interpretation are not permitted.
Figure 12 contains diagrams of various neck features which would not be considered "plain," such as a putter which has multiple necks and a putter with a neck which is too long.