Last year on the 17th green at Waialae Country Club was the last—and only—time Kevin Na opened a green-reading book on the golf course.

It was there that he needed a birdie-eagle finish to shoot 59 in the third round of the Sony Open. However, after reading the green for his 10-footer, he glanced at the book, adjusted his read, took the shot, and missed.

He wound up winning the tournament with a 61, but nuts to the book, he thought.

“It didn’t work out too well,” Na recalled Thursday, “so that was the last time I saw it.”

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Na’s longtime caddie, Kenny Harms, used to use the book to verify his initial read before the PGA Tour cracked down on green-reading materials.

The opening round of the Sentry Tournament of Champions marked the first day of the Tour’s new rule—which went into effect January 1—which says players and caddies are only allowed to use committee-approved books.

The rule aims to ensure that reading putts was the product of skill, judgment, and feel, in addition to quality preparation. Handwritten notes will be allowed, though no devices can be used to obtain data.

“I probably bury my head in them too much as it is,” said Joel Dahmen. “I don’t use them at home and I putt ok, so there’s no real reason to have them out here for me anyway.”

Collin Morikawa and Brooks Koepka noted they used books to read putts but also to identify where to land approach shots given the green’s slopes and contours.

“It’s a little more work for the caddie on Tuesday and Wednesday for them to get the slopes and the grain out there,” Dahmen said.

Perhaps the player who most affected by the rule is the notoriously data-driven Bryson DeChambeau, who ranked as a top-35 putter on Tour in each of the past four seasons but does not yet own a top-10 finish at the Masters—the only tournament that doesn’t permit the use of green books.

“At the end of the day, I still go based off of my intuition most of the time,” DeChambeau said. “I look at something, ‘ok, I think it looks a little like this.’ The times where I’ve putted best have been where my intuition is matched up with reality, and what it’s actually doing because sometimes (the books) can be wrong.”

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