grips must have the same cross-section shape along their entire length (the diameter can vary), and with the exception of the putter, must have a circular cross-section. The putter may have any cross section that is symmetrical along the length of the grip through at least one plane; "shield" profiles with a flat top and curved underside are common. Grips may taper from thick to thin along their length (and virtually all do), but they are not allowed to have any waisting (a thinner section of the grip surrounded by thicker sections above and below it) or bulges (thicker sections of the grip surrounded by thinner sections). Minor variations in surface texture (such as the natural variation of a "wrap"-style grip) are not counted unless significant. Re-gripping Advances in materials have resulted in more durable, longer-lasting soft grips, but nevertheless grips do eventually dry out, harden, or are otherwise damaged and must be replaced. Replacement grips sold as do-it-yourself kits are generally inexpensive and of high quality, although custom grips that are larger, softer, or textured differently from the everyday "wrap"-style grip are generally bought and installed by a clubsmith. Re-gripping used to require toxic, flammable solvents to soften and activate the adhesive, and a vise to hold the club steady while the grip was forced on. The newest replacement kits, however, use double-sided tape with a water-activated adhesive that is slippery when first activated, allowing easier installation. Once the adhesive cures, it creates a very strong bond between grip and shaft and the grip is usually impossible to remove without cutting it off. Hosel The hosel is the portion of the club head to which the shaft attaches. Though largely ignored by players, hosel design is integral to the balance, feel and power of a club. Modern hosels are designed to place as little mass as possible over the top of the striking face of the club, which lowers the center of gravity of the club for better distance. Club head Each head has one face which contacts the ball during the stroke. Putters may have two striking faces, as long as they are identical and symmetrical. Some chippers (a club similar in appearance to a double-sided putter but having a loft of 35–45 degrees) have two faces, but are not legal. Page 135 of the 2009 USGA rules of golf states: The club head must have only one