The one ball rule — same brand and model, all round
The one ball rule requires you to play the same brand, make and model of ball for the entire round. It is an optional local rule (Model Local Rule G-4) that committees adopt for elite competitions — it is not part of the standard Rules of Golf.
The short answer
- One ball rule = same brand, make and model for the whole round.
- It's a local rule, not a standard rule — check the terms of competition.
- A Titleist Pro V1 and Pro V1x are different models — you can't mix them.
- Breach = general penalty (two strokes in stroke play, loss of hole in match play).
- Outside the one ball rule you may substitute balls freely when a rule allows.
What the one ball rule requires
The one ball rule is Model Local Rule G-4, adopted at the committee's discretion. When it is in effect, you must play with the same brand, make and model of golf ball you started the round with.
The USGA's own example is the clearest: if you start with a Titleist Pro V1, you must play a Titleist Pro V1 for the rest of the round. You may not switch to another brand, and you may not even switch to another model of Titleist — the Pro V1x does not qualify, because brand, make and model must all match.
What you can do is replace a ball with a new one of the same brand, make and model at any point. The rule restricts the type of ball, not the number of balls.
Where the one ball rule applies
| Level of play | One ball rule? |
|---|---|
| PGA Tour, DP World Tour and other elite professional events | Yes — usually written into the tournament local rules (often with a Pro-Am exception) |
| Amateur championships and elite events | Sometimes — depends on the committee's terms of competition |
| Club competitions and medals | Rarely |
| Casual rounds with friends | No — not in effect unless your club says so |
This is why most golfers have never encountered it. If you turn up to a regular weekend medal you can play whatever ball you like, including swapping mid-round when a rule allows you to substitute.
The penalty for breaching it
Where a local rule is in place and the breach does not state its own penalty, the general penalty applies: two strokes in stroke play and loss of hole in match play.
In practice, the penalty is applied for playing a stroke with a ball that is not the required brand, make and model. Committees running elite events typically add a further safeguard: players are given an opportunity to correct the mistake by replacing the wrong ball with a conforming one, and repeated breaches escalate to disqualification.
Ball marking matters more under this rule
Because the ball is constrained by type, players in these events mark balls carefully and keep track of which model is in play. If you ever play an event where the one ball rule might apply, confirm it before the round and check your ball model on the 1st tee.
What you can still do without the one ball rule
Outside the one ball rule, the Rules of Golf allow ball substitution in many common situations, including when you take relief under most rules, when your ball is unfit for play, or when your ball is lost or out of bounds and you are playing from a different place. You may also change the type of ball you play between holes.
So the practical answer for 99% of golfers: play whatever ball you want, whenever you want, unless your committee has explicitly adopted Model Local Rule G-4.
Frequently asked questions
What is the one ball rule in golf?
Can you change golf balls during a round?
Can you switch from a Pro V1 to a Pro V1x under the one ball rule?
What is the penalty for breaking the one ball rule?
Is the one ball rule in the Rules of Golf?
Equipment rules, decoded
One ball rule, 14-club limit, broken clubs, replacing a ball that is unfit for play — the Save Strokes card covers every equipment call you'll face, with the rule number and the penalty right there.