Ply (game theory)

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In two-player sequential games, a ply refers to one turn taken by one of the players.

In standard chess terminology, one move consists of a turn by each player; a ply in chess is a half-move. Thus, after 20 moves of a chess game, 40 ply have been completed, 20 by white and 20 by black. 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.

It is thought that good human chess players can often look 10 or more plies ahead.^{[citation needed]} In computer chess the concept of ply is 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. Levy and Newborn estimate that each additional ply adds 50 to 70 points (Levy & Newborn 1991:192). The Deep Blue chess computer which defeated Kasparov in 1997 would typically search to a depth of between six and twelve plies to a maximum of forty plies in some situations.

Arthur Samuel coined the term as a back-formation based on the word reply in his paper on machine learning in 1959.^[1]