Players and caddies are constantly looking at them, but what's inside?
What Are Green Reading Books?
The weekend golfer will usually be happy enough to find the green in regulation and walk off with a par.
Whether they're short or long, most club golfers will take being on the putting surface with a birdie chance - even if that chance is only slim.
For pros, it's different.
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The best in the business benefit from a little more information.
In fact, many of them rely on it, as do their caddies.
So, what are green reading books? And why are they not allowed at The Masters?
Numbers, number, numbers...
Lots of numbers, probably too many for some golfers - those who just prefer to aim and fire, and not over analyse.
Players like data-driven Bryson DeChambeau, however - and 'The Golf Scientist' is certainly not the only one - absorb every little detail.
It's because they feature detailed graphics of every hole, denoting the direction and degree of the slopes, which doesn't just give players help when they are actually on the green, but assists them with their approach shots.
It might be just a couple of degrees, or it could be a much more severe slope - it's all welcome information.
Armed with the degrees of slope from everywhere on the green, which are highlighted with lots of arrows, it makes life a lot easier.
Not everyone approves of green reading books.
Ian Poulter has been critical of them in the past, stating on Twitter a few years ago, "Too much time wasted in a book."
It's a contentious issue, with some arguing that they contribute to slow play in golf.
Phil Mickelson has previously said that greens books allow him to do 80 per cent of his read before he gets to the green, and he labelled the slow play argument as "idiotic".
Why Are Green Reading Books Banned At The Masters?
DeChambeau is definitely a player who doesn't want to see green reading books abolished, although because they're now allowed at Augusta National - a place which sets its own rules - he, like everyone else, is having to work things out for himself during Masters week.
And this, he says, is "very hard".
"Growing up in college, we didn't have greens books, and I played well then," said the pre-tournament favourite on the eve of the event.
"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 they can be wrong. The greens books can be wrong."
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