Appendix II, 4c of the 2008 edition of the Rules of Golf states that:

"The design, material and/or construction of, or any treatment to, the clubhead (which includes the face) must not … have the effect of a spring which exceeds the limit set forth in the Pendulum Test Protocol on file with the R&A"

This is a new Rule which applies to all golfers of all ability and in all forms of play.

Since the announcement of this new Rule in August 2002, golfers have been encouraged to check the conformance status of their driver, and any driver they may buy in the future, in preparation for its introduction. Many drivers which were previously conforming have now become non-conforming as a result of the new Rule.

To help provide correct information on driver head conformance, two Lists have been compiled and published:

To view or search the List of Conforming Driver Heads, please click on the link below. All of the clubs on this List conform to the 2008 edition of the Rules of Golf.
 

To view a list of drivers which became non-conforming on 1 January 2008, please click on the link below:
 

The above lists are maintained jointly by The R&A and the United States Golf Association (USGA). Any post-1998 drivers which do not appear on either of these Lists should not automatically be assumed to be either conforming or non-conforming without further investigation by the player. Such drivers may not have been submitted to The R&A or the USGA for a ruling or they may have been ruled non-conforming for another reason. In the event of any doubt over the conformance status of a club not included on either of these Lists, please contact the manufacturers of the club or The R&A.

IMPORTANT NOTE: Appendix I, Part C, 1a states that "If the Committee wishes to limit players to drivers that have a clubhead identified by model and loft, that is on the Conforming Driver Head List, the List should be made available and the following condition of competition used:

"Any driver the 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 R&A.

Exception: A driver with a clubhead that was manufactured prior to 1999 is exempt from this condition."

Notes on Administration of Conforming List as Condition of Competition:

1. The penalty for breaching this Condition can be found in Appendix I, Part C, 1a.
2. The List of Conforming Driver Heads Condition of Competition is only recommended for use by Committees in charge of competitions restricted to highly skilled players (including top amateur events). However, the Condition should not be confused with the Rule itself. The Rule applies to all golfers whereas the Condition requires that the driver the player uses not only conforms to this Rule, but is also included on the List of Conforming Driver Heads. The Condition is an additional requirement which can be introduced by Committees in charge of elite level competitions. It is not recommended for use at Club level golf.
3. The Condition only covers driver heads. However, any driving club that a player carries must conform to all other rules pertaining to golf clubs.
4. It is the player's responsibility to carry clubs that conform to the Rules of Golf, and all relevant Conditions of Competition. Players and manufacturers, therefore, should take care to ensure that any modification of a driver head does not alter the playing characteristics such that it becomes different from the sample submitted for evaluation which appears on the List.
5. If a driver fails any on-site evaluation (i.e.; club length, spring effect, groove dimensions, etc.), the individual club is considered non-conforming and may not be carried by the player even if that model of driver appears on the List.
6. Until further notice, there will be no expiration date for a clubhead included on the List. However, a clubhead included on the List may be removed if it no longer conforms to the Rules of Golf due to, for example, a subsequent Rules change.

To view a list of Frequently Asked Questions on the application and implications of this new Rule and the condition, please click on the link below:
 

To view the procedure for measuring the flexibilty of a golf clubhead, please click on the link below.