Imagine you’re playing the 15th at Augusta. Most of us have dreamed of that in the past. You’ve gone through the green and face a devilishly difficult chip back towards the flag with the water lurking on the other side of the sloping green. Your playing partner’s ball is six feet past the hole as you're playing, almost directly on your line. It would be rather a nice safety cushion to leave it in place, to act as a potential backstop if your pitch is slightly heavy handed. It could prevent you going in the water! The question though is, are you allowed to leave it there?
Rule 15.3a says that if a player reasonably believes that a ball on the putting green might help anyone’s play (such as by serving as a possible backstop near the hole), the player may: Mark the spot of the ball and lift it if it is their own ball, or if the ball belongs to another player, require the other player to mark the spot and lift the ball.
If you know a ball is going to act as a potential backstop then you should have it marked. And, if your playing partner knows their ball is a potential backstop, they should mark it.
If you’re playing match play then you can agree with your opponent to leave a ball in place on the putting surface as the benefit that may come will only affect your match.
But, in stroke play you cannot make an agreement with a playing partner to leave a ball in place to act as a backstop. If you do make a such an agreement and a stroke is played with the helping ball in place (whether there’s a collision or not) both players who made the agreement receive the General Penalty of two shots.
It doesn’t matter if the players who made the agreement aren’t aware it’s against the Rules to make such an agreement. They still receive the General Penalty.
If nobody says anything, there is no agreement and no breach of the Rules. But The R&A offers guidelines on how to prevent “backstopping” in their committee procedures ,with a view to protecting the whole field in a stroke play competition. The guidelines say, “if there is a reasonable possibility that a player’s ball close to the hole could help another player who is about to play from off the green, both players should ensure that the player whose ball is close to the hole marks and lifts that ball before the other player plays.”
The situation arose in the playoff for this year’s BMW PGA Championship at Wentworth. On the first extra hole, Billy Horschel was about to play from a greenside bunker towards the pin and the water, with Thriston Lawrence’s ball acting as a potential backstop. Chief referee Mark Litton stepped in, stopping Horschel from playing until Lawrence had marked.
Basically then, in stroke play you should not deliberately use another player’s golf ball as a backstop, nor should you leave your ball in a place where it could be a help to your playing partners.
