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Many videos on YouTube channel "Kingscrusher" which discuss the play of new neural-net engines like Leela Chess Zero focus considerable attention upon the development of "thorn pawns" which are pawns that have reached the sixth rank and are blocked by a pawn in front but do not have pawns ahead of them to either side. In my limited study of the game in decades past, I've seen great significance given to passed pawns, but never any special attention given to thorn pawns. Do such pawns have a significance that has only been fully brought out as a result of AI-assisted research, or have they always been understood but simply not in things I'd read? Or is Kingscrusher giving them more significance than they deserve?

A typical thorn-pawn situation would occur when Black has pawns at f7, g7, and h7, white moves h5, Black g6, and white pushes h6. If Black has castled kingside, that pawn at h6 can be very hard to capture, and its presence kills the escape square at g7 and the possibility of easily establishing one at h7. Was that a common pattern in years past, and if so what was it called?

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    When Leela castles King's side, it's only in order to engage the King aggressively in the pawn storm. To the degree the military asks AI what to do, they get the answer to attack now, attack all in, all out, always attack now. (I prefer dimwitted generals not understanding the situation). Pieces are protected by the edge of the board, it almost halves the angles from which they can be attacked. Isn't Leela also noted for NOT castling too often? Because attackers are more easily defeated in the all accessible center. The E1 King with his back to the wall is positioned to attack in the endgame.
    – LocalFluff
    Dec 19, 2018 at 12:47

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I had never heard the term, but such pawns are very important for their contribution to certain mating patterns, the most famous of which would be Damiano's mate, which was included in Damiano's 1512 book Questo libro e da imparare giocare a scachi et de li partiti. Here's an archetypal example:

[FEN "5rk1/6pQ/6P1/8/8/8/8/8 w - - 1 1"]
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  • Nice historical example. As used by LC0, however, thorn pawns more typically seem to exhibit their nastiness by controlling territory and paralyzing opposing pieces. An example of that pattern would eliminate the white queen from the above example, put the black rook somewhere else on the eighth rank, and have a white rook placed so it can move to the eighth rank from many places where it could also wreak other mischief. Often, Leela will fall be behind on the quantity of material on the board, and yet--because of thorn pawns--be ahead on the quantity of active material.
    – supercat
    Dec 13, 2018 at 18:51
  • Kingscrusher just added another video of a game where the thorn pawn is so powerful that Kingscrusher refers to the concept as a major undiscovered "cheat code" of chess. See youtu.be/Z90V_CkFWK4 on youtube. Five minutes into the video, LC0 is significantly behind in material, but ends up establishing a thorn pawn that totally dominates.
    – supercat
    Dec 16, 2018 at 8:55

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