Thinking inside the box"Plain" and "unconventional" are not exactly opposites, but between the two terms lies a vast gray area where USGA officials and creative club designers dance a curious tango usually reserved for the courtship ritual of the Madagascar hissing cockroach. In short, the USGA has established a creative limit at "plain in shape," and designers are trying to stretch the meaning of that phrase as much as they can. Although the game's rules-makers want golf clubs to continue to look like golf clubs, designers know that reaching some of the USGA's prescribed performance limits might require unconventional thinking -- and geometry.The USGA in recent years has tried to define the meaning of "plain in shape," stating most recently that clubs must fit within a five-inch-by-five-inch frame and cannot have "cavities in the outline of the heel and/or the toe of the head that can be viewed from above" or "severe or multiple cavities in the outline of the back of the head that can be viewed from above."Yet designers, particularly from smaller companies, continue to challenge what's traditional, searching for ways to improve clubhead stability -- and approach the USGA's limit on moment of inertia of 5,900 grams-centimeters squared -- and to distinguish their designs from others."The USGA wants a club to look like something that had its heritage in this game," says Alan Hocknell, Callaway's vice president of innovation and advanced design. "The rule is of interest to us, but it hasn't directly stopped us from doing anything."Of course, by establishing the MOI limit of 5,900, the USGA has set a goal for manufacturers, and many contend that "plain in shape" can't get you there. "There's only so much more you can do," says Jeff Summit, technical director at Hireko, which has introduced the pentagonal-shaped Power Play Caiman. "The geometry does help. I see how truly difficult it is to get to that number. Even in that five-by-five frame, I still can't achieve it unless I remove some material somewhere."The challenge for club designers -- and those governing the gray area -- is how much and where.