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.

Related Topics:
Computer chess - Game tree - Elo - Deeper Blue - Kasparov

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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