Let’s direct our attention to the diagram presented below:
I deliberately didn’t mark the last move as usual. The question is, can you find the shortest possible sequence of moves that could lead to the position on the diagram? This task is called Proof Game (or, in this case, Shortest Proof Game), and it belongs to the category of retrograde analysis chess problems.
Let’s initiate the solution process by examining the absent chess pieces and the likely sequences leading to their capture:
- Black’s side is lacking two pawns and a knight, while White’s only captured piece is a knight. It is reasonable to deduce that the white knight must have executed three captures before being taken by Black.
- Unlike White, Black has the theoretical potential to make moves with other pieces, as the bishops, queen, and king are unimpeded by the absence of two pawns. However, any such move would have to be followed by returning to the initial position, making it highly improbable for these pieces to be part of the shortest feasible solution. Consequently, we can reasonably assume that only pawns and knights were involved in the actual moves.
We can start reconstructing the scene. As the last move was made by Black, and all black pieces remain in their starting positions, it must have been a knight’s move (since other pieces were ruled out). This knight undoubtedly captured its white counterpart on the b8 square. Likewise, the initial move in the game was executed by the white knight to f3, ensuring that the knight had a clear path to b8.
1. ♘f3 … ♞xb8
We must also take into consideration that the white knight was responsible for capturing two black pawns on its way to b8. To complete this task without wasting too many moves, there is only one possible solution:
1. ♘f3 e5 2. ♘xe5 … 3. ♘xd7 … 4. ♘xb8 ♞xb8
Good! We are almost there. It isn’t difficult to find the missing moves of the black knight, as there is only one way to fill the gaps:
It is fair to add that the task is a version by Andrei Frolkin of a problem by Ernest Clement Mortimer, and was published in Shortest Proof Games (1991). As it is a relatively simple example of retrograde analysis, I will try to come up with something more difficult in the future articles.