Ply

To represent a level in the tree the term "ply" was introduced by Arthur Samuel in his

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paper on machine learning at 1959 (Samuel-1959.PDF).

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Consider the word 'reply' - the response, or comeback, to an utterance. You might think of it as the re-ply to an initial ply.

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In chess, ply refers to a half-move: one turn of one of the players. Thus, after 20 moves of a chess game, 40 ply have been completed, 20 by white and 20 by black.

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It is suggested that good human chess players can often look 10 or more ply ahead.

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In computer chess the concept of ply becomes important because one ply corresponds to one level of the game tree. It has been found that an increase in search depth of one ply corresponds on the average to an increase in playing strength of approximately 80 Elo points. The Deeper Blue chess computer which defeated Kasparov in 1997 could search to a depth of 12 ply.

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In the game of go, by contrast, a ply is the normal unit of counting moves; so for example to say that a game is 250 moves long is to imply 250 ply.

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See also:

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• Minimax algorithm
• Computer chess

Computer chess: The idea of creating a chess-playing machine dates back to the eighteenth century. Around 1769, the chess playing automaton called The Turk became famous before being exposed as a hoax. After that, the field of mechanical chess research languished until the advent of the digital computer in the 1950...

Game tree: In game theory, a game tree is a directed graph whose nodes are positions in a game and whose edges are moves. The complete game tree for a game is the game tree starting at the initial position and containing all possible moves from each position....

Elo: ELO or Elo has several meanings:...