Can you use a green-reading book during a round?

Yes, you can use a green-reading book or app during a round, but Rule 4.3a limits how detailed and large those materials can be. Your ability to read the green yourself must remain an essential part of putting.

What the rule actually says

Rule 4.3a says you can't use equipment that artificially eliminates the need for a skill essential to the game. Interpretation 4.3a/1 applies this to green-reading materials: detailed putting green maps and similar electronic or digital aids are allowed, but their size and scale are restricted so that reading the green stays a skill you have to use, not something the book does for you. In practice, this means pocket-sized books with general slope arrows and basic contours are fine. Oversized, highly detailed maps that show every subtle break at a scale that lets you just trace the line are not.

When the committee can shut it down

A committee can adopt Model Local Rule G-12, which modifies Rule 4.3a to prohibit the use of any materials for reading the line of play on the putting green. If G-12 is in effect, you can't use a green-reading book, a phone app with green maps, or even a handwritten note with slope details. Check the local rules sheet or the notice board before you tee off.

Exceptions that change the answer

• The committee adopts Model Local Rule G-12 → All green-reading materials are prohibited for strokes from the putting green — books, apps, notes, anything that helps you read the line.

Frequently asked questions

What counts as too detailed or too big?

The rule doesn't give exact dimensions, but the principle is that the material can't replace your own judgment. A pocket guide with general slope arrows is fine; a full-page, high-resolution map showing every subtle break at a scale where you can just trace the line is not.

Can I write my own notes on green slopes during the round?

Yes. Information you gather yourself during the round — like noting how a putt broke — is allowed. You just can't process or interpret that data with a device during the round to get a recommended line.

What's the penalty if I use a banned green-reading book?

First breach: general penalty (two strokes in stroke play, loss of hole in match play). Second breach: disqualification.

Does this apply to yardage books with green diagrams?

Yes. Any material — paper or digital — that helps you read the line of play on the putting green falls under the same size and scale limits, and the same G-12 prohibition if the committee adopts it.