Gelback, Cleveland Golf"s lightweight, visco-elastic, vibration-damping material, is placed directly behind the hitting area of the clubface in the back cavity. * Breakthrough milling technology creates pristine edges increasing "ZIP" and shot consistency-especially from poor lies. Conforms to 2010 USGA groove rule. * Maximizes allowable groove dimensions while conforming to 2011 USGA groove requirements. * Gelback is vibration-tuned to remove unwanted vibration on mis-hits while still allowing the player necessary feedback from the clubhead for a soft yet responsive feel.