The short answer
- One ball rule = same brand and model for the whole round.
- It's a local rule, not a standard rule — check the terms of competition.
- Two models from the same maker count as different balls, so you can't mix them.
- Breach = one penalty stroke for each hole where you're in breach.
- 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 and model of golf ball you started the round with.
That means one entry on the List of Conforming Golf Balls. Two models from the same maker count as different balls, and so do different colours with identical markings. You may not switch to another brand, or to another model from the same maker.
What you can do is switch to another ball of the same brand and model between holes, or during a hole when a Rule allows substitution. The rule restricts the type of ball, not the number of balls.
Where the one ball rule applies
G-4 is recommended only for competitions limited to highly skilled players, meaning professional and elite amateur events. It isn't in effect in a club competition or a casual round unless the committee adopts it, so check the terms of competition.
For most club and casual play it doesn't apply. In a regular weekend medal you can play any conforming ball, change between holes, and substitute mid-hole when a Rule allows it.
The penalty for breaching it
Model Local Rule G-4 sets its own penalty: one penalty stroke for each hole during which you're in breach.
The penalty is for making a stroke at a ball that isn't the required brand and model. If you've dropped, placed or replaced a wrong ball but haven't played it yet, you can fix it without penalty (Rule 14.5). Once you discover a breach, you must switch back to your starting brand and model when you play from the next teeing area, or you're disqualified. If you find out mid-hole, you may finish the hole with that ball or place a correct one on the same spot.
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 cut or cracked during the hole (not just scratched or scraped), 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 for most club and casual play: play any conforming ball, change between holes, and substitute during a hole only when a Rule allows it. Otherwise you hole out with the ball you teed off with. G-4 only matters when your committee has adopted it.
Frequently asked questions
What is the one ball rule in golf?
Can you change golf balls during a round?
Can you switch between two models from the same maker 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?
Rules at a glance
Save Strokes: Golf Rules Quick Reference is a PDF covering 20 rules situations, with a printable bag card and a phone card.