a. Plain in Shape

The "plain in shape" requirement in Appendix II, 4a originates from the "traditional and customary" requirement in Appendix II, 1a. It is generally a descriptive rule, although in reality, it is challenging to define exactly what a golf club can or should look like. The following section assists in explaining what is and what is not permitted, but it should be noted that this is not an all-inclusive list. Even if a clubhead satisfies all of the points outlined below, there may still be features or characteristics which render it not generally plain in shape. An overall assessment of the appearance of the head should always be made.

The general provision of Rule 4a, Appendix II provides that:

This provision basically means that the design of the clubhead must be free from gimmicks (though putters are viewed more liberally than woods and irons), must have the general appearance of a clubhead, must not have the appearance of another object, or incorporate certain features which are designed to resemble another object (see Figure 16). All parts of the head must be rigid throughout their length, including permanent, permissible appendages. As a general guideline, "rigid" means that it must not be possible to bend or flex any part of the clubhead by hand (see Figure 17).

Rule 4a, Appendix II acknowledges that defining whether a clubhead is "plain in shape" is not easy. However, the USGA modified the wording in 2008 to help better define what is intended by this Rule. The Rule now reflects the more liberalized application for putters, which has evolved over the years and provides more detail regarding what is and is not permitted for iron heads and woodheads.

Rule 4a, Appendix II goes on to state that:

Holes Through the Face

Holes through the face are not permitted for any club (see Figure 18).

Holes Through the Head

Facsimiles of Golf Balls

This provision is fairly straightforward. Generally, features that are spherical or semi-spherical in nature, whether or not they contain dimples, may be considered to be facsimiles (see Figure 22 for examples of conforming and non-conforming designs). However, other features may be considered facsimiles and are evaluated on a case-by-case basis.

Features for the Purpose of Meeting Dimensional Specifications

Appendix II, 4b requires that, for all clubs, the distance from the heel to the toe of the clubhead must be greater than the distance from the front to the back (see Design of Clubs, Sections 4b(i), (ii) and (iii)). Clubs which incorporate features that are designed to or have the effect of circumventing this requirement are not permitted (see Figure 23).

Features That Extend Into or Ahead of the Face

Features That Extend Significantly Above the Top Line of the Head

Furrows or Runners

Furrows and/or runners are deemed to extend into the face if the edge of the face has any concavity (point of inflection or turning point). This can be determined by placing a straight edge along the edge of the face.

The application of this Rule is fairly straightforward in most cases and can be determined, as noted above, simply by placing a straight edge along the leading edge of the face and looking to see if there is a gap between those two edges. However, woods and hybrids commonly have a smooth transition between the face and the rest of the body, which can sometimes make it difficult to determine, in a repeatable and reproducible manner, whether features “extend into” or intersect with the face.

Where the transition between the body and the face is not clearly defined (i.e., the face does not meet the body at a sharp edge or chamfer), the point of intersection shall be defined as the point where a line inclined at 45° from the reference plane is tangent to the cross-section (see Figure 29).

• A reference plane is defined as the plane tangent to the face at the center of the impact area.
• A vertical cross-section is created by establishing a plane perpendicular to the reference plane, running through the clubhead at the area of interest (i.e., furrow or runner).

Any substantial concavities in said profile are considered sufficient evidence that a runner or furrow extends into the face and thus does not conform with Appendix II, 4a(i).

Where the transition between the body and face is clearly defined by a chamfer of at least 45° with respect to the face, furrows and/or runners are permitted to intersect the chamfered surface, provided the feature does not intersect the face itself (see Figure 32).

Optical and Electronic Devices

Clubheads that incorporate - for example, prisms, mirrors, reflective materials, light beams, metronomes or mechanical devices such as spirit levels, or other similar devices - are not permitted (see Figures 33 and 34).

Electronic devices in or on the club shaft or grip, which have the sole purpose of identifying the club, may be permitted. The identification information is restricted to:

Any such device must meet all other Rules and associated guidelines and must not vibrate or emit light or sound. If the device is capable of any function other than identification, the golf club will be considered not traditional and customary in form and make (see Section 1a(i) and, therefore, non-conforming.

NOTE: Any system used in conjunction with a club incorporating such a device must comply with the Local Rule Permitting the Use of Distance Measuring Devices (see Note to Rule 14-3 of the Rules of Golf and Appendix I, Part B Section 9) and the USGA-R&A Joint Statement on Electronic Devices.

Rule 4a, Appendix II states additional specifications for woods and irons:

Cavities in the Outline of the Heel and/or Toe

This provision applies to cavities in the outline of the heel and/or toe when "viewed from above." When making this assessment, viewed from above is interpreted to mean the range from directly above the head to the normal address position for that club. This restriction does not apply to horizontal cavities around the skirt of the head, which might be visible from above (e.g., horizontal slots or indentations such as the speed slot (see Figures 32a and b)).

Severe or Multiple Cavities in the Outline of the Back of the Head

A severe cavity is one in which the entrance of the cavity is narrower than its width at any other point. Multiple cavities are not permitted (see Figures 35c and d). Cavities in the crown of the head are permitted, even if they are primarily designed for sighting, aiming or head alignment, or to accommodate markings for such aids (see Figure 35e). However, cavities in the crown of a driver head are filled for the purposes of evaluating the head's volume (see Design of Clubs, Section 4b(i)).

Transparent Material

Features Extending Beyond the Outline of the Head

Any fin, knob, plate or other appendage which protrudes beyond the outline of the head is not permitted for any purpose.

Note: While this provision is not applied to putters, the USGA has determined that unusual features which protrude beyond the outline of the heel and/or toe of the putter head are usually considered not generally plain in shape or not traditional. However, as previously noted in other parts of this section, other permanent appendages to the putter head are permitted, provided that:

b. Dimensions, Volume and Moment of Inertia

Appendix II, 4b is divided into three categories - woods, irons and putters. The volume and moment of inertia limits apply only to woodheads.

Please refer to "The Rules of Golf" for a description and illustrations of where these measurements should be made. When performing such measurements in the field, the best method is to use a pair of calipers. For the heel to toe measurement, a rigid, straight edge can be held upright against the extremity of the toe end.

Volume Limit:

This Rule was introduced in 2004 due to the trend of increasing driver head sizes. The Equipment Standards Committee determined that woodheads larger than those already permitted were not traditional and customary. Many woodheads in the marketplace incorporate markings on the sole or other part of the head to indicate approximate volume. However, there is a fairly simple method of measuring clubhead volume, which is based on the displacement of water. By submerging a clubhead in a large measuring container, partially filled with water, the amount that the water rises indicates the head's volume.

A more accurate method, but not that much more complicated, is the use of a similar container of water placed on a digital weighing scale. With either method, the head is submerged to a point just above the crown (i.e., the hosel is not included).

Archimedes' Principle states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object - and since water has a specific gravity of 1.0, this means that 1 cubic centimeter of water has a mass of 1 gram. Therefore, the container of water should be placed on the scale and the weight should be set to zero. When the head is submerged in the water, the weight displayed on the scale (in grams) is equivalent to the volume of the head (in cubic centimeters).

In situations where a club is marked with a "cc" value which is in excess of the Rule (i.e., above 460 cubic centimeters), the Committee's policy is to rule that the club is non-conforming - regardless of the actual volume measurement. This is to avoid confusion in the marketplace.

Prior to measuring the volume of a clubhead, the head should be inspected for cavities. All cavities on the crown should be filled with waterproof clay or other similar material using a straight-line method which connects the edges of the cavity. The straight-line method does not follow the taper or curvature of the surface of the head, rather the cavity is filled so that it becomes a flat surface which adjoins the outer edges.

Only significant cavities in the sole will be filled. We define significant as any cavity or series of cavities which have a collective volume of greater than 15cc.

Moment of Inertia (MOI) Limit:

The MOI of a driver head is a measurement of its resistance to twisting and, therefore, it is one indication of the ‘forgiveness' of a clubhead.

The measurement of MOI is one of only a few limits within the Rules which cannot be easily performed in the field. This is because the MOI measurement requires specialized equipment and the head must be removed from the shaft. However, drivers with higher MOI measurements are generally represented by models manufactured in recent years and, as a result, have been routinely submitted to the USGA or R&A due to the publication of the List of Conforming Driver Heads.

The MOI of a driver head is directly related to the head's weight, as well as to the distribution of the head's weight. As a result, a driver head which is designed to be adjustable for weight must conform to the Rules in all available configurations (see Design of Clubs, Section 1b). Additionally, a player who adds additional weight to the driver head, such as with the addition of lead tape, must be certain that the driver head still conforms to the Rules (see Rule 4-1b). To assist golfers with this determination, the Committee will notify a manufacturer that has submitted a driver head having an MOI which is close to the limit, to advise its customers that the addition of any weights to that particular head, including lead tape, other than the weights supplied by the manufacturer for that particular head, is not permitted as it would likely render the club non-conforming.

In practice, due to the shape and size of iron heads, this Rule is rarely violated. It is retained, in part, to help maintain the traditional shape by which irons are recognized. However, it appears that hybrid irons are becoming more common and, as a result, this Rule may have even greater utility in the future.

The following illustrations demonstrate the dimensional specifications for putters:

The Rule goes on to describe how these measurements should be made for traditionally shaped heads and that for unusually shaped heads, the heel-to-toe measurement may be made at the face. Given the dimensional restrictions listed above, which help to control the size and shape of a putter head, the clause regarding unusually shaped heads is now rarely, if ever, applied.

It is important to note that appendages are not permitted if they are for the purpose of meeting the dimensional specifications above.

c. Spring Effect and Dynamic Properties

Effective January 1, 2008, the USGA re-worded and re-positioned the spring effect clause previously found under Rule 5a, Appendix II to this section of the Rules. Among other things, the language codified the underlying principles of several decisions made by the USGA over the past several years regarding spring design features. Rule 4c, Appendix II now states:

Because "spring effect" is purely a performance-related limit which cannot be easily measured in the field without specialized equipment and cannot be assessed through visual inspection of the clubhead, the USGA compiles and maintains a List of Conforming Driver Heads. The List is updated on a weekly basis and can be accessed through the USGA's Web site (www.usga.org). Additionally, features in the clubhead design that are intended to or have the effect of unduly influencing the clubhead's spring effect are not permitted. The USGA has adopted an interim test method to assist in streamlining evaluation of these types of designs and, as a result, if the spring effect of the clubhead, measured at any point on the face, does not exceed the limit set forth in the Pendulum Test Protocol on file, the clubhead would likely conform to this provision.

For competitions involving expert players, the USGA recommends that the Committee in charge of the competition adopt the List of Conforming Driver Heads as a Condition of Competition (See Appendix I, Part C, Section 1a). The Condition requires that any driver a player carries must have a clubhead, identified by model and loft, that is named on the current List of Conforming Driver Heads issued by the USGA. It is important to note that a driver with a clubhead manufactured prior to 1999 is exempt from the requirement of the condition.

While this Condition is not recommended for use by other competition committees, the Rules regarding club conformance, including spring effect, apply to all rounds conducted under USGA Rules of Golf, including those rounds played for the purpose of posting a handicap score.

Finally, if a manufacturer makes claims of spring-like features or that a clubhead, other than a putter, exceeds the Pendulum Test limit, or if there is strong evidence to suggest that the club, in fact, exceeds the limit, then the club will be considered non-conforming to this Rule.

d. Striking Faces

Appendix II, 4d states that:

The exception for putters was introduced in order to accommodate traditional blade‑type putters.

Determining whether a surface constitutes a second (or third) striking face is often a matter of opinion, However, in general, a surface should be considered an additional striking face if:

All three of the putters illustrated in Figure 38 would be ruled non‑conforming.

The addition of lead tape to the back face of a putter with two conforming striking faces would not be contrary to the Rules. Additionally, cosmetic/decorative markings on one of two permissible surfaces, that do not affect performance, will not usually create a different striking face.