Abstract: Most golf greens slope, which causes balls that are putted on them to follow curves, rather than moving in straight lines. An heuristic visual device is provided to assist a golfer in estimating how a putted golf ball will "break" on a sloping green. The invention provides graphic representations of trajectories of putted golf balls rolling on greens that slope solely along principal directions (i.e. front-to-back and right-to-left). A golfer, who must initially estimate how the green on which he is planning to putt actually slopes in the two principal directions, can metally combine those graphically represented putting trajectories in order to estimate the effects of gravity and thereby to predict his proper `line of play`.