Consider the word 'reply' - the response, or comeback, to an utterance. You might think of it as the re-ply to an initial ply.
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.
It is suggested that good human chess players can often look 10 or more ply ahead.
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 200 Elo points. The Deep Blue chess computer which defeated Kasparov in 1997 could search to a depth of 12 ply.
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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A ply can also refer to a strand or layer of material, such as yarn, rope, plastic, wood, or paper. Plywood gets its name from this.