Can I use a green-reading app on my phone during a round?

No, you can't use an app that reads the green for you — that's interpreting slope and recommending a line, which Rule 4.3a prohibits. A simple distance app that only gives yardages is allowed, but anything that shows break, aim points, or heat maps crosses the line and costs you two strokes in stroke play or loss of hole in match play.

What the rule actually says

Rule 4.3a lets you use equipment for distance and directional info — think a rangefinder or a GPS that tells you 146 to the middle. What you can't do is use equipment that "artificially eliminates or reduces the need for a skill or judgment essential to the challenge of the game." Reading a green is one of those essential skills. So an app that draws the break, gives you an aim point, or color-codes slope is doing the reading for you. That's the same as pulling out a detailed green map bigger than the allowed scale, and it's a breach. The clarification 4.3a/1 spells it out: electronic or digital materials that help you read your line on the putting green are limited in size and scale. A phone screen showing a detailed contour map with a recommended line blows past that limit.

Exceptions that change the answer

• The Committee adopts Model Local Rule G-12 → All green-reading materials — paper or digital — are banned for strokes from the putting green. Even a basic yardage book with green contours becomes prohibited.

Frequently asked questions

Can I use the GolfLogix / 18Birdies / Green Books app on the green?

Only if you restrict it to yardages only. Any feature that shows break, aim points, slope percentages, or a recommended putt line makes it a prohibited green-reading material under Rule 4.3a and clarification 4.3a/1. Two-stroke penalty in stroke play, loss of hole in match play.

Is a paper yardage book with green contours allowed?

Yes, if it meets the size and scale limits in clarification 4.3a/1 — roughly a 3/8-inch-to-5-yards scale (about 1:480) and no larger than a 7" x 7" page. Many commercial green books exceed this and are non-conforming. Check the book's specs or ask the Committee.

What if my playing partner uses a green-reading app — can I look at it?

Looking at someone else's prohibited material isn't a breach for you, but you can't ask them to interpret it for you. And if they share the info voluntarily ("aim two cups left"), they've breached Rule 10.2a (advice) and you've received advice — two strokes each in stroke play.