4.a(1) - Plain in Shape
The “plain in shape” requirement originates from the ‘traditional and customary’ requirement in Section 1a. It is purely a descriptive provision, although in reality it can be challenging to define exactly what a golf club can or should look like. The following sections assists in explaining and illustrating what is and what is not permitted, but it should be noted that the examples that follow are not all-inclusive. Additionally, 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.
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 golf clubhead as opposed to another object and must not incorporate features which are designed to resemble another object (see Figure 23). All parts of the head (including permanent, permissible appendages) must be rigid throughout their length. As a general guideline, “rigid” means that it must not be possible to bend or flex the head or its parts by hand. (see Figure 24).
The provision acknowledges that defining whether a clubhead is “plain in shape” is subjective. To better clarify the provision and its interpretation, it is split into two categories – one which covers “all clubs” and the other which covers the additional specifications relevant only to “irons and woods”. The Section also reflects the more liberalized application for putters which has evolved over the years and provides more detail regarding what is and what is not permitted for iron heads and woodheads.
4.a(2) - Holes through the Face
Holes through the face are not permitted – see Figure 25.
4.a(3) - Holes through the Head
• Holes through the head are not permitted for wood heads – see Figure 26.
• Holes through the head are not permitted for iron heads. However, features within the cavity back of an iron clubhead that form a hole or holes may be permitted, (e.g. support bars), provided that the feature is contained within the outline of the main body of the head and the hole cannot be viewed from above – see Figure 27 a/b.
• This provision is interpreted very liberally for putters and holes through the head (excluding the face) for any purpose including aiming, sighting or alignment are permitted – see Figure 28 for examples of permissible holes through the head of a putter.
4.a(4) - Features for the purpose of meeting Dimensional Specifications
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 Section 4b, below). Clubs which incorporate features which are designed to or have the effect of circumventing this requirement are not permitted – see Figure 29.
4.a(5) - Features that extend into or ahead of the face
Permitted features in or on the main body of the head must not extend into the face of an iron or wood club. This would include alignment features on the crown – see Figure 30. Alignment lines which have been engraved or inscribed onto an iron clubhead would generally be permitted.
• Certain features are permitted to extend into the face of a putter, including alignment features and concavities (or furrows) on the crown (see Section below on Furrows and Runners). However, such features must not have a depth or height greater than 0.25 inches (6.35 mm) when measured against the top line of the face.
• Features of any nature that extend ahead of the face are not permitted on any club – see Figure 31.
4.a(6) - Features above the top line of the head
• For putters, alignment or other features must not extend above the top line of the face by more than 0.25 inches (6.35 mm) – see Figure 32.
• For woods and irons, features which otherwise meet the requirements for “plain in shape” must not extend above the top line of the head by more than 0.1 inch (2.54 mm).
• Permanent or semi-permanent lines or other markings which have been painted, inscribed or otherwise incorporated (see Section 1a) for alignment purposes are permitted.
4.a(7) - Furrows and Runners
• Furrows or runners which extend into the face of a wood or iron club from any part of the head are not permitted – see Figure 33.
• While this provision is applied strictly for furrows or runners which may appear on the sole of a putter, exceptions may be made for other parts of the head – see Figure 34.
Furrows and/or runners are deemed to extend into the face if the leading edge of the face has any concavity (point of inflection or turning point).
The application of this provision is fairly straightforward in most cases and can be determined simply by placing a straight edge along the leading edge of the face and looking to see if there is a gap between these 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 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 35).
• A reference plane is defined as the plane tangent to the face at the centre of the impact area.
• A vertical cross-section is created by establishing a plane perpendicular to the reference plane, usually running through the centre of the face/clubhead and the area of interest (i.e. furrow or runner).
Projecting the points of intersection so defined onto the reference plane may be used to define a profile (see Figure 36).
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 Section 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 37).
If a runner has been chamfered back away from the face, by at least 45°, then it would not be considered to extend into the face – see Figure 38.
4.a(8) - Optical and Electrical Devices
Clubheads which incorporate prisms, mirrors, reflective materials, light beams, metronomes or mechanical devices such as spirit levels are not permitted – see Figures 39 and 40.
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:
(a) the club’s owner, such as address and phone number;
(b) inventory tracking information;
(c) detection of the club’s use during a round.
Any such device must meet all other requirements of the Equipment Rules 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 device or application used in conjunction with a club incorporating such a device must comply with the provisions of Rule 4.3 of the Rules of Golf.
4.a(9) - Cavities in the Outline of the Heel and/or Toe
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. The restriction does not apply to horizontal cavities around the skirt of the head, which might be visible from above – see Figures 41a and b.
4.a(10) - Severe or Multiple Cavities in the Outline of the Back of the Head
• Severe cavities (as viewed at address), which go all the way through the head, are not permitted. A “severe” cavity is one where the entrance to the cavity is narrower than its width at any other point – see Figure 41c.
• A single cavity in the back of the head is permitted – see Figure 41b. However, multiple cavities are not permitted – see Figure 41d.
• Cavities in the crown of the head are permitted, even if they are designed primarily as an aid to sighting, aiming or aligning the swing plane or the head position, or to accommodate markings for such aids – see Figure 41e. However, all cavities on the crown are filled for the purpose of measuring the volume of the head – see Section 4b(i)).
4.a(11) - Transparent Material
• Clubheads made entirely of transparent material are permitted.
• Transparent material that is added to an otherwise non-plain head does not render the head “plain in shape”. For example, a wood head which has a vertical hole from the top surface through to the sole would be ruled non-conforming (see Section 4a(i) and Figure 26). Filling this hole with a transparent material (e.g. perspex or glass), would not alter this ruling.
4.a(12) - Features Extending Beyond the Outline of the Head
Any fin, knob, appendage or plate which is protruding beyond the outline of the head is not permitted, whatever the purpose.
NOTE: While this provision does not apply to putters, The R&A and USGA have determined that unusual features which protrude beyond the outline of the toe and/or heel of a putter head may be ruled not “plain in shape” or not “traditional and customary”. However, as previously noted, other permanent appendages to the putter head are permitted provided that:
• the feature is rigid throughout its length (i.e., cannot be bent or flexed by hand);
• the feature does not extend forward of the face; and,
• the features does not extend above the top line of the face by more than 0.25 inches (6.35 mm).
4.b(1) - Dimensional specifications
When performing these measurements in the field, the best method would be to use a pair of callipers. For the heel to toe measurement, a rigid, straight edge (e.g. a ruler) should be held upright against the extremity of the toe end.
In practice, due to the shape and size of iron heads, this provision is rarely encroached. It is retained in the Equipment Rules, in part, to help maintain the traditional shape by which irons are recognized. However, while most irons are still relatively narrow from front to back, the popularity of hybrid clubs means that this provision has greater utility today.
Given all of the dimensional restrictions for putter heads, which help define the size and shape, the additional clause regarding unusually shaped heads is rarely, if ever, applied.
It is important to note that appendages are not permitted if their only purpose is to make the clubhead meet the dimensional specifications described in this provision (see Section 4a(iv) and Figure 29).
4.b(2) - Volume limit
In practice, many of the larger headed clubheads in the market place have a marking somewhere on the head indicating the approximate volume of the head (this is the cubic centimetres or “cc” value).
Fortunately, for clubs where there is no indication of volume, there is a fairly simple method of determining the actual volume of a clubhead in the field and it is broadly based on Archimedes’ Principle and the displacement of water. All that is needed is a large measuring container, half filled with water. The measure of clubhead volume would be the amount by which the water level rises once the clubhead has been submerged into the water. Therefore, if the container is filled with 1 litre of water and the level rises to 1450 millilitres when the head is submerged to the base of the hosel – this would mean that the head has a volume of 450 cubic centimetres.
The official test protocol for measuring volume is a more accurate method, but not that much more complicated, and it requires a similar container of water placed on a set of digital weighing scales.
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 centimetre of water has a mass of 1 gram. Therefore, the container of water should be placed on the scales and the weight should be set to zero. When the head is submerged in the water, the weight displayed on the scales (in grams) is equivalent to the volume of the head (in cubic centimetres).
In situations where a club is marked with a “cc” value which is in excess of the Rule (i.e. above 460 cubic centimetres), The R&A’s and USGA’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 to create a ‘straight line’ which connects the edges of the cavity. The ‘straight line filling technique’ 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 concavities on the sole will be filled, meaning any cavity or series of cavities which have a collective volume of greater than 15 cc.
4.b(3) - Moment of Inertia (MOI)
The MOI test is a measurement of a clubhead’s resistance to twisting and, therefore, it is an indication of its ‘forgiveness’.
The measurement of MOI is one of the few limits within the Equipment Rules which cannot easily be performed in the field. This is because the testing equipment is very specialized and it can only be measured by de-shafting the head (the hosel remains on the head for the purpose of the test). However, high MOI is only associated with modern, hollow, high volume driver heads and, due to the publication of the List of Conforming Driver Heads (see Rule 4c below), most of these clubs are now routinely submitted to the Governing Bodies for a ruling – so that they can be included on this List.
As the MOI of a driver head is affected by a change in its weight and the position of the centre of gravity, a driver which is designed to be adjustable for weight must conform to the Equipment Rules in all configurations (see Rule 1b). Moreover, when adding additional weight to a driver (e.g. with lead tape), the player must be certain that the club is still within the limit. To assist golfers with this determination, The R&A and USGA have developed a policy whereby if a driver head is submitted for a ruling and it is measured to have an MOI which is close to the limit, the manufacturer will be encouraged to advise its customers that the addition of any other weights to that model (including lead tape), other than the weights which were supplied by the manufacturer, is not permitted as it would likely render the club non-conforming. Additionally, the manufacturer must take care over its claims and must not advertise that the product is over the limit for MOI.
Given that the “spring-effect rule” is purely a performance related limit which cannot be measured easily in the field without specialized equipment and cannot be assessed through a visual inspection of the clubhead, The R&A and USGA have compiled and maintain a List of Conforming Driver Heads. This List can be accessed via The R&A’s or USGA’s website and it is updated weekly.
For competitions involving highly skilled players, Model Local Rule may be introduced requiring players to use a driver which is included on this List. This Local Rule is an additional requirement for these players, which means that, not only must their clubs conform to the Equipment Rules, but the model and loft must also be included on the List of Conforming Driver Heads. This Local Rule is not recommended for use at other levels of golf. However, it is the player’s responsibility to ensure that his or her driver conforms to the Equipment Rules if it is not on the List.
As well as meeting the requirements of clause (i) in the above Rule, clubs are also tested for compliance to clause (ii) using the Pendulum at points on the face other than at the centre and, depending on the results, other tests may be carried out. Any club which is found to include a feature that is designed to act like a spring, independent of the level of flexibility achieved by the design, could be ruled non-conforming.
Finally, if claims of “spring like” qualities are made by the manufacturer which suggest that a club is manufactured to be in excess of the limit, or there is evidence to suggest that the club is indeed over the limit, then the club would be deemed to be non-conforming.
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 interpretation. However, in general, a surface should be considered an additional striking face if:
the area is flat and it is clearly designed to be used for striking the ball; or
it is opposite the intended face and consists of a flat surface of a different loft and/or material; or
it is a flat surface on the toe and/or heel of a cylindrical, rectangular or square head design which could effectively be used to strike the ball; or
it could otherwise effectively be used to strike the ball.
All three of the putters illustrated in Figure 45 would be ruled non‑conforming.
The addition of lead tape to the secondary face of a dual-faced putter would not be contrary to the Equipment Rules. In addition, cosmetic or decorative markings on one of two permissible surfaces, that do not affect the performance, will not usually create a different striking face in breach of this provision.