What happens if you use a device to read the green?

You can't use a gadget that tells you the break or gives you a recommended line on the putting green — that's a breach of Rule 4.3. First offense is the general penalty (two strokes in stroke play, loss of hole in match play); a second breach gets you disqualified.

What the rule actually forbids

Rule 4.3a says you must not use equipment that artificially eliminates or reduces the need for the skill and judgment essential to golf. On the green, that means no device that measures slope, interprets the break for you, or spits out a recommended line of play. A plain distance-measuring device or a compass is fine — they only give yardage or direction. But the moment the tool tells you "aim here" or "this putt breaks two inches left," you've crossed the line. The same restriction covers detailed green-reading books or digital maps. The R&A/USGA clarification (4.3a/1) caps the size and scale of any putting-green map you carry during a round. If your committee adopts Model Local Rule G-12, even those limited maps are banned.

What you're still allowed to do

You can absolutely use your putter as a plumb line — hold it up, let gravity show you the general slope. That's considered part of the player's skill. You can also consult notes you made before the round, old yardage books, or anything you gathered prior to teeing off. Recording stats or heart-rate data during the round for later review is permitted too; you just can't process that data mid-round to get club recommendations or line suggestions. Weather info from forecasts is allowed. Measuring temperature and humidity at the course is allowed. What you can't do is measure wind speed on the course or use powder, a handkerchief, or a ribbon to judge the breeze.

Penalties escalate fast

First breach of Rule 4.3: general penalty — two strokes in stroke play, loss of hole in match play. Second breach, unrelated to the first: disqualification. That second strike doesn't have to be the same gadget or the same hole; any separate breach of Rule 4.3 after you've already been penalized once ends your round. So if you use a banned green-reading app on the 3rd green and get caught, then pull out a bubble level on the 12th, you're DQ'd.

Exceptions that change the answer

• Committee adopts Model Local Rule G-12 → Even the limited-size green-reading materials normally allowed under Rule 4.3a/1 are prohibited for the competition.
• Committee adopts Model Local Rule G-5 → All distance-measuring devices are prohibited for the competition.

Frequently asked questions

Can I use my phone's level app to check the slope on the green?

No. A bubble level or any digital level on your phone is prohibited equipment under Rule 4.3a(1)/1. Using it earns you the general penalty on the first breach and disqualification on a second.

Is a printed green-reading book legal?

Yes, but only if it meets the size and scale limits in Clarification 4.3a/1. Oversized or highly detailed maps that effectively give you the break are not allowed. Check with the committee — they may have adopted Model Local Rule G-12 banning all such materials.

Can I use a rangefinder with slope on the green?

No. A rangefinder that displays elevation change or plays-like distance is measuring slope, which Rule 4.3a(1) explicitly forbids. Turn the slope function off or leave the device in the bag while on the putting green.

What if I just mark my ball with a line and aim it using the green's contour?

That's perfectly fine. Aligning your ball's own marking by eye is a skill, not equipment assistance. The breach in Interpretation 4.3a/2 happens only when you use an external alignment device to show the line, then position the ball to match it.

Does the penalty apply if I didn't know the device was illegal?

Yes. Rule 4.3 doesn't require intent. If the equipment is used in a prohibited way, the penalty applies regardless of whether you knew it was banned. That's why it's worth checking the local rules sheet before you tee off.