Ply (game theory)

In two-player sequential games, a ply refers to one turn taken by one of the players. The word is used to clarify what is meant when one might otherwise say "turn".

The word "turn" can be a problem since it means different things in different traditions. For example, in standard chess terminology, one move consists of a turn by each player; therefore a ply in chess is a half-move. Thus, after 20 moves in a chess game, 40 plies 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 plies.

The word "ply" used as a synonym for "layer" goes back to the 15th century.[1] Arthur Samuel first used the term in its game-theoretic sense in his seminal paper on machine learning in checkers in 1959,[2] but with a slighty different meaning: the "ply", in Samuel's terminology, is actually the depth of analysis ("Certain expressions were introduced which we will find useful. These are: Ply, defined as the number of moves ahead, where a ply of two consists of one proposed move by the machine and one anticipated reply by the opponent"[3]).

In computing, the concept of ply is important because one ply corresponds to one level of the game tree. The Deep Blue chess computer which defeated Kasparov in 1997 would typically search to a depth of between six and sixteen plies to a maximum of forty plies in some situations.[4]

See also


  1. ^ Online Etymology Dictionary, "ply" (cited 24 April 2011)
  2. ^ A.L. Samuel, March 3, 1959: Some Studies in Machine Learning Using the Game of Checkers (cited 25 August 2006)
  3. ^ A.L. Samuel, March 3, 1959: Some Studies in Machine Learning Using the Game of Checkers, p. 601 (cited 2 May 2018)
  4. ^ Murray Campbell, et al. 2002. Deep Blue

Further reading

  • Levy, David; Newborn, Monty (1991), How Computers Play Chess, Computer Science Press, ISBN 0-7167-8121-2

External links

  • The dictionary definition of ply at Wiktionary

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.