Understanding the rule regarding ‘green reading materials’ in golf

The Model Local Rule (MLR) G-11 is making headlines this Sunday, December 3, due to the penalty stroke received by Collin Morikawa during the third round of the 2023 Hero World Challenge. Morikawa received two penalty strokes for violation of this measure.

MLR G-11 is called Restricting Use of Green-Reading Materials and was issued by the United States Golf Association (USGA), with the concurrence of the R&A. It internally amends Rule 4.3a of The Official Rules of Golf, which deals with the permitted and prohibited uses of equipment by players.

MLR G-11 specifies the following (via USGA):

"During a round, the player may use only the yardage book(s) approved by the Committee. This restriction also applies to any other maps of the course, including hole-location sheets.

Additional restrictions apply to handwritten notes and any other materials used by the player that could help read the line of play on the putting green:

Handwritten notes may be added to an approved yardage book or approved hole-location sheet before or during the round by the player or the player’s caddie only and are limited to information gathered by the player or caddie only.
Handwritten notes may contain information gained only through personal experiences of the player or the player’s caddie from the course or by watching a televised broadcast, but are limited to information gained:

While observing a ball that was rolled or played (whether by the player, caddie or someone else) or
Through the player's or the caddie's feel or general observations of the putting green."

That is, the rule allows the use of handwritten notes during the round but prohibits the use of devices to obtain the information contained in the handwritten notes.

Model Local Rule G-11 came about in 2021. According to Golf Digest, the PGA Tour stated the following as the purpose of its formulation at that time:

"[To] return to a position where players and caddies use only their skill, judgment and feel along with any information gained through experience, preparation and practice to read the line of play on the putting green."

What was Morikawa's Restricting Use of Green-Reading Materials rule violation?

According to PGA Tour official Stephen Cox, as reported by Sports Illustrated, Collin Morikawa's caddie, J. J. Jakovac, used handwritten notes on hole four of the third round of the 2023 Hero World Challenge. The data contained in these notes was allegedly obtained with the use of an unauthorized device.

Collin Morikawa was informed of his penalty prior to the start of the fourth round. NUCLR GOLF later reported citing Golf Channel reporter Todd Lewis that Matt Fitzpatrick was the one who reported Morikawa to event officials.

Morikawa finished the 2023 Hero World Challenge in 7th place with a score of 12-under 276.