Proof game

A proof game is a legal game that starts from the initial position and reaches a target diagram. Its basic purpose is to demonstrate that the position is reachable. It therefore belongs naturally with retrograde analysis, where the board's previous history is part of the problem.

In composition, a PG stipulation, with PG short for Proof Game, commonly asks the solver to reconstruct a game that reaches the diagram in the stated length. Both sides may make moves that would look strange in competitive play, because the problem's objective is not to win the game but to reach the required position.

Terminology varies across sources. In retrograde analysis, proof game can mean any legal route from the initial position to the diagram. Some composition sources use proof game for a shortest construction, while Shortest Proof Game, or SPG, makes the minimality requirement explicit. The stated stipulation therefore determines what must be proved.