What is a proof game?
A proof game asks a simple question: here is a position, now show me a legal game that reaches it.
The rules of the genre
In a proof game you are given a final position and a number of moves. You must find a game, starting from the normal opening position, that reaches exactly that position in exactly that many moves. When the number of moves is the smallest possible, the problem is called a shortest proof game (often written SPG).
A well-made proof game has one solution only. That is what turns it from a puzzle into a proof: every move is forced by the evidence on the board, and no other order works.
Why they are harder than they look
- The moves do not have to be good. Pieces get captured for no reason, kings walk into the open, and queens are given away.
- The obvious explanation is often wrong. A missing piece might not have been taken by the piece that seems to be standing on its square.
- Some moves leave no trace. A piece can move out, capture something and return, so the final position shows only the missing victim.
- The move count is strict. With three moves each, one wasted move ruins the whole sequence.
An example
Three moves each, from the opening position, reach this:
Black’s king stands on f7, Black’s f-pawn is gone, and White has lost the light-squared bishop. Black needed two of its three moves for e5 and Bc5, so the f-pawn never moved: it was captured on f7, and the king then took the piece that captured it. The missing bishop is the only white piece that could have reached f7 in time:
1. e4 e5 2. Bc4 Bc5 3. Bxf7+ Kxf7
Proof games every day
Every case on Three Moves Ago is a short proof game: three moves for White, three for Black, one solution. Some start from the opening position, and the harder ones start from a real middlegame.