This laser fires down two lines - a line pointing to your target, and a line along the face of the putter, square to the target line * Improve your putting almost instantly! Since the Laser creates lines, it"s easy to see if the putter moves off-target. * Easily snaps on the front of the putter shaft with a twist of the wrist * Create vivid visual images for recall * For use indoors only. Each set of batteries lasts up to two hours of continuous use.