Richard, it's usually the visual counterbalance in the surrounding area to the slope of the green.
The first time I played Baltusrol Upper I was in a tournament with a shotgun tee off.
My first hole was # 3.
I hit a good iron onto the green and read the putt with a break of a few feet.
When I asked the caddy where I should aim, he walked up about 15 feet above the hole.
My first thought was, "it's 8:00 am, and they gave me caddy who's been drinking for three hours."
So, I added a little to my initial read.
After I hit and watched the putt, I said, "God Damn, I never saw that, maybe he's sober as a judge"
I marveled at how some greens broke more and some greens broke far less than I read.
The surrounds were what through me off, they could make a green appear to break in one direction, when it did just the opposite, or, alot more.
If you used a level, you would see the true slope and understand how you had been deceived by the backround.
It's like that exercise where the line is the same length, but the way the end of the line is configured, visually, leads you to conclude that one is longer/shorter than the other
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