Retrograde Analysis - Example

Example

An example of a retrograde analysis problem is shown at right. The solver must deduce White's last move. At first blush, there seems to be no solution: on any square from which the white king could have moved, it would have been under a seemingly impossible double check; however, thinking more we can discover that if white king moved from f5, then the black move before that could be pawn f4xg3, taking the white pawn on g4 en passant. Thus before f4xg3, white must have played pawn g2-g4. But what did Black move before that? The white king on f5 was under check by the bishop on h3 and there was a white pawn on g2. The only possibility is that black moved knight g4-e5 with discovered check. Therefore White's last move was king f5 takes knight on e5. (The entire sequence of moves is thus 1...Ng4-e5 discovered check 2.g2-g4 f4xg3 double check 3.Kf5xe5.)

Note that in this example the actual next move is essentially irrelevant; Black has a choice of several relatively trivial ways of delivering instant checkmate. (E.g. Qf3-d5#, Rd6-d5#, etc.).

One might ask: "If the white pawn was on g2 from the start of the game, how could the Black Queen come to be on f3, also a Black Bishop to be on h3? Is this problem valid?" The problem is indeed valid. The initial position has to be legal, but not particularly reasonable. That the Black Queen and Bishop were en prise in the initial position may be disappointing to some, it does not invalidate the problem.