A player's ball is at rest. He rests his club on the grass immediately behind the ball and the ball moves. What is the ruling?
If the grass is compressed to the point where it would support the weight of the club, the club is considered grounded. Therefore, the player has addressed the ball and Rule 18-2b applies. The player incurs a one-stroke penalty and must replace his ball unless it is known or virtually certain that some other agency (e.g., a dog or wind) caused the ball to move.
If the grass had not been compressed to the point where it would support the weight of the club, the player has not grounded his club and, therefore, has not addressed his ball. The player incurs no penalty under Rule 18-2b, but he is subject to penalty under Rule 18-2a if the player's actions caused the ball to move.