What happens if I hit my shot while another ball is still on the green helping me?

Yes, there's a penalty. In stroke play, if you agree with another player to leave their ball on the green as a backstop and you make your stroke with it still there, you both get two penalty strokes — even if you didn't know it wasn't allowed.

When the penalty applies

Rule 15.3a only covers a ball at rest on the putting green. If another player's ball is sitting near the hole and could act as a backstop for your chip or putt, you're supposed to ask them to mark and lift it. If you both agree to leave it there and you play your shot anyway, that's the breach. The penalty is two strokes for each player who made the agreement, and it doesn't matter whether you knew the rule or not.

Match play is different

In match play, you and your opponent can agree to leave a ball in place to help one of you. Since the outcome only affects your match, the Rules allow it. No penalty, no problem — but only in match play. The moment you're in a stroke-play competition, that same agreement costs you both two strokes.

What counts as "agreeing"

It doesn't have to be a formal handshake. If you ask someone to leave their ball, or they offer and you say nothing and just play, that's an agreement. Even a nod or staying quiet when they say "I'll leave it" counts. The Rules treat any understanding between players to keep a helping ball in place as the breach.

Exceptions that change the answer

• Match play → Players may agree to leave a ball in place to help the opponent; no penalty.
• Ball not on the putting green → Rule 15.3a doesn't apply; interference is handled under Rule 15.3b instead.

Frequently asked questions

What if I didn't know leaving the ball was against the rules?

Doesn't matter. In stroke play, the penalty applies even if neither player knew the agreement wasn't allowed. Rule 15.3a/1 makes it clear: lack of knowledge is not a defense.

What if the other player's ball wasn't actually going to help me?

If you reasonably believed it might help — like a ball near the hole that could stop your ball — the rule applies. It's about your reasonable belief at the time, not whether it actually would have helped.

Can I just play first instead of asking them to lift it?

In stroke play, the player required to lift their ball may play first instead. But if you've already agreed to leave it as a backstop and then play, the penalty is already triggered — playing first doesn't undo the agreement.

What if my ball hits their ball that was left on the green?

You play your ball as it lies, no penalty for the collision (Rule 11.1a). But if there was an agreement to leave it as a backstop, you still get the two-stroke penalty under Rule 15.3a for the agreement itself.

Does this apply to ball-markers too?

Yes. Rule 15.3c says if a ball-marker might help or interfere, you can move your own or require the other player to move theirs. Same principle — you can't agree to leave a marker in place to help in stroke play.