Ply

A ply counts one move by one side: either a White move or a Black move. The written sequence 1. e4 e5 therefore contains two plies. e4 is the first and e5 the second, while move number 1 groups them in ordinary game notation.

The word removes an ambiguity in move. In casual speech, move may mean one player's action or a complete White-and-Black pair. Databases and software use ply when the count must advance one side at a time. A game that ends immediately after White moves can therefore have an odd number of plies.

The term also appears in chess engine analysis. A chess engine is a program that calculates positions. A depth of six plies follows six individual moves along a line. If every branch were searched to the same depth, this would equal three complete move pairs, although modern engines may extend or reduce parts of their search.