I played last week with a fellow who had a new TM 3 metal head club. This club had a deep groove behind the clubface from the bottom but the groove could not be seen from the top. He said that there was no rule regarding the 'trampoline effect' on any club but the driver. But in fact, the 'driver' is not defined in the rules book, that i can find. In only one place do i even find the word 'driver' ; in Rule 4, 1a, note. In Appendix II there are 3 classes of clubs mentioned, namely woods, irons and putters.
So how would you define a 'driver' and can anyone make a club with a high trampoline face which nominally avoids the driver definition? The player in question loved his new club and hit the 3 wood 280 on average.