REINSTATEMENT
Joaquim Crusats and Andrey Frolkin
Two friends, who always adhered strictly to the laws of chess, were playing a normal game, with no time limit, when suddenly a series of small earthquakes began. The players left the house, but after a while, as nothing else happened, they continued the game. There was enough time only for each one of them to make a new move and then a new series of earthquakes began; so they wisely decided to leave the game for the next day. When the game was resumed, they found the following position on the board:
The dialogue between White and Black went as follows:
B: Whose move is it?
W: Yours; I remember that I moved last.
B: I have completely forgotten the moves we made yesterday.
W: Me too, but never mind, I took note of the fact that it was your turn to move.
B: Alright then, I think I can easily win this one.
W: Wait a moment! Now I realize that yesterday the earthquakes shook my bishops off the board. I should have seen the irregularity right away, I am sorry.
B: You are right; your bishops are both on the floor. It doesn't really matter though, because even with your two bishops on the board, regardless of wherever they were, I see no way you can win the game.
W: On the contrary, my dear friend, I perfectly remember where my bishops were and I can unambiguously prove that I will win this game.
Where were the white bishops?
Solution:
W: Consider the official chess rule 7.5 concerning irregularities: “If during a game it is found that pieces have been displaced from their squares, the position before the irregularity shall be reinstated (…).” Does this help?
B: I see; we abandoned the game for a while during the first series of earthquakes, and it was only when we came back that we made our last moves. We both have to retract our last move and reinstate the position with both your bishops on the board.
W: The positions of my bishops will prove that your last move was an illegal castling.
B: But you know that I always abide by the rules!
W: Indeed you do. Your castling was apparently legal because not all my units were on the board. After the correct position is reinstated you are then free to choose any other legal move you want. But I am afraid that you won’t be able to parry my threat of mate by promoting on h8.
B: So your bishops had to be shaken off from c8 and d8: this implies that I moved both my king and rook in my last move.
W: Good try! But this is impossible. Remember that I also made a move after the earthquakes. My last move was not made with my pawns – my dark-squared bishop had to exit from his homebase; nor did I move my king – because it would have been in check; the bishops on c8 and d8 would show that your last move was the “illegal” castling, not a check to my king. Notice that my king couldn’t move from f7 – because if you castled, my king couldn’t be there. I wouldn’t have had any possible last move. No, the solution is different: my bishops were initially on a6 and a7. One of my bishops had to be on a7 so that your last move could not have been …Sa7-b5 instead of castling. In this way your king could not have been put in check before the first series of earthquakes took place and you cannot claim that once my wBa6 had vanished you simply forgot to parry the check and played any other move instead. Moreover, your king couldn’t come from b8, moving inadvertently into check, because my bishop a7 couldn’t have checked him. Notice also that the balance of the position prevents your last move being …RxSd8, QxSa5, SxRb6, or …BxRd7, since my a- and e-pawns could not promote and were thus captured on their files.
B: Well, your bishops on a6 and a7 seem to prove that my last move was the “illegal” castling. But first of all you must retract the last move you made yesterday and, as you explained before, once proved that my last move was castling, you don’t have any possible move of that sort!
W: My last move is perfectly determined and it was obviously made with one of my bishops.
B: But your bishops were not on the board!
W: No, my dear friend, my bishop a7 was still on the board when you castled. It was shaken off from b8 in the second series of earthquakes. Another game?
