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".

"Turn" is problematic since it means different things in different traditions. For example, 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.

Arthur Samuel coined the term as a back-formation based on the word "reply" in his paper on machine learning in 1959. [ [http://www.research.ibm.com/journal/rd/441/samuel.pdf A.L. Samuel, March 3, 1959: "Some Studies in Machine Learning Using the Game of Checkers"] (cited 25 August 2006)]

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 twelve ply to a maximum of forty ply in some situations.

ee also

*Minimax algorithm

References

*Citation
surname1=Levy|given1=David|authorlink1=David Levy (chess player)
surname2=Newborn|given2=Monty
year=1991
title=How Computers Play Chess
publisher=Computer Science Press
ID=ISBN 0-7167-8121-2