Some more discussion here https://www.chess.com/forum/view/general/what-is-the-opening-with-the-most-book-moves-in-chess, including one with 18 book moves https://www.chess.com/openings/Ruy-Lopez-Opening-Marshall-Attack-Modern-Main-Line-Spassky-Variation
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Any move that follows conventional theory might be labeled as a "book move" by the opening database in Chess.com. The main lines of the Closed Ruy Lopez (for example), runs through 6.Re1 b5 7.Bb3 d6 8.c3 0-0. And, I guess, 9.h3
After that, I am not sure there is any theory that is so common that Chess.com considers it a "book move", even though some people claim the "main line" extends for another six moves. I personally don't see it.
There are book lines of 20-25 moves in the Ruy Lopez and the Sicilian.
Jan 14, 2022
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#5
In my "Kids, don't try this at home!" game, the first new move came on move 28. Everything before that was known theory.
Jan 14, 2022
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#6
blueemu wrote:
In my "Kids, don't try this at home!" game, the first new move came on move 28. Everything before that was known theory.
28 moves theory? 
thats around half of an average game
Jan 14, 2022
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#7
EuweMaxx wrote:
blueemu wrote:
In my "Kids, don't try this at home!" game, the first new move came on move 28. Everything before that was known theory.
28 moves theory? thats around half of an average game
I gave a link to it above, if you're curious.
Mar 17, 2023
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#9
But for me, I found out
that there's 18 book moves, and that's all. It's called the Ruy López Opening: Marshall Attack, Modern, Main Line, Spassky Variation
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