Can I use a green-reading book or device during a round?

Yes, you can use a green-reading book or device, but only if it meets the size and scale limits in Rule 4.3a/1. A Committee can also ban them entirely with Model Local Rule G-12.

What Rule 4.3 allows for green reading

Rule 4.3a lets you use equipment to help your play, but you can't use anything that artificially eliminates the need for skill or judgment essential to the game. For green-reading materials, Interpretation 4.3a/1 limits the size and scale of detailed putting green maps and any similar electronic or digital materials. The idea is that your ability to read a green must remain an essential part of putting. So a small book with general contours is fine; a large, highly detailed map that does the reading for you is not.

When a Committee can ban them completely

A Committee may adopt Model Local Rule G-12, which modifies Rule 4.3a to prohibit the use of any materials for reading your line of play on the putting green. If that Local Rule is in effect for the competition or the course, you cannot use any green-reading book, device, or notes at all. Always check the notice board or the tournament's Local Rules sheet before you pull out a green-reading aid.

Alignment devices and other prohibited uses

You also can't use an alignment device — any object manufactured to help align the ball — to show the line of play and then position your ball based on it. Interpretation 4.3a/2 makes clear that marking your ball with an alignment device aimed at the hole, then rotating the ball to match, is a breach. Using a bubble level, a weighted string, or a bottle as a level to gauge slope is also prohibited under Interpretation 4.3a(1)/1.

Exceptions that change the answer

• Model Local Rule G-12 is in effect → All green-reading materials are prohibited for strokes from the putting green.
• You use an alignment device to show the line of play and position your ball accordingly → You are in breach of Rule 4.3a (general penalty: two strokes or loss of hole).

Frequently asked questions

How big can my green-reading book be?

There's no single size limit in the Rule, but Interpretation 4.3a/1 says the Committee can restrict the size and scale of detailed putting green maps. In practice, most Committees follow the R&A/USGA guidance: a book no larger than 4.25 x 7 inches (10.8 x 17.8 cm) with a minimum scale of 3/8 inch to 5 yards (1 cm to 5 m).

What if the tournament bans green-reading books?

If Model Local Rule G-12 is in effect, you cannot use any green-reading material — book, device, or handwritten notes — for strokes from the putting green. Using one gets you the general penalty (two strokes in stroke play, loss of hole in match play).

Can I use my putter as a plumb bob to read slope?

Yes. Interpretation 4.3a(1)/1 explicitly allows using your club as a plumb line to assist in judging slope and contours. You just can't use a separate bubble level, a weighted string, or a bottle as a level.

What's the penalty for using a banned green-reading book?

The general penalty under Rule 4.3: two strokes in stroke play (added to the hole where the breach happened) or loss of hole in match play. If you use it on multiple holes before being caught, you get the penalty for each hole, up to a maximum of four strokes per round in stroke play.