Under promotion to a bishop.
Posted: Sun Dec 09, 2018 6:09 pm
We all know that when a pawn reaches the eighth rank it is normally promoted to a queen. There are rare cases where this would produce stalemate so a rook is preferred. There are even rare cases where promotion to a knight is preferred if it can produce an immediate mate in a situation where no other piece can do this. However, I have never read about a pawn being promoted to a bishop so I set myself a problem. Could I devise a position where promoting a pawn to a bishop is the most sensible move. I think I have done this. Consider the following position.
FEN 5bkr/4p2p/1P2P2P/8/2p5/8/8/R1K5
(I hope that I have got the FEN notation correct - if not white, to play has Ra1, Kc1, and pawns b6, e6, h6 and black has Rh8, Kg8, Bf8, and pawns c4, e7, h7.)
White can see that, unless he prevents it, black's next move will be Bxh6+, followed by Kg7, thus freeing his rook. To prevent this he plays Ra8 and black now plays his only legal move c3. White now plays b7 and black must play c2. We now have this position. (This was the one I originally devised, but I wanted to show how it could have occurred.)
FEN R4bkr/1P2p2p/4P2P/8/8/8/2p5/2K5
Black now has no legal move so if white wants to play for a win, his next move must give black a legal move. There are two ways to do this - move the rook off the eighth rank or promote the b pawn to a minor piece. If he chooses the former, then black will play Bxh6+ followed by Kg7 freeing his rook as mentioned above. I suggest that from this position the result of the game is not clear.
Now consider white playing b8=B. Black can only move his bishop and will play Bxh6+ but after Kxc2 if he plays Kg7 it is met by Be5+ and black goes a rook down. His only alternative is to move the bishop again and I think that wherever it goes white has a winning reply.
Do you agree with this analysis? Have I found a position where promoting a pawn to a bishop is the best possible move?
FEN 5bkr/4p2p/1P2P2P/8/2p5/8/8/R1K5
(I hope that I have got the FEN notation correct - if not white, to play has Ra1, Kc1, and pawns b6, e6, h6 and black has Rh8, Kg8, Bf8, and pawns c4, e7, h7.)
White can see that, unless he prevents it, black's next move will be Bxh6+, followed by Kg7, thus freeing his rook. To prevent this he plays Ra8 and black now plays his only legal move c3. White now plays b7 and black must play c2. We now have this position. (This was the one I originally devised, but I wanted to show how it could have occurred.)
FEN R4bkr/1P2p2p/4P2P/8/8/8/2p5/2K5
Black now has no legal move so if white wants to play for a win, his next move must give black a legal move. There are two ways to do this - move the rook off the eighth rank or promote the b pawn to a minor piece. If he chooses the former, then black will play Bxh6+ followed by Kg7 freeing his rook as mentioned above. I suggest that from this position the result of the game is not clear.
Now consider white playing b8=B. Black can only move his bishop and will play Bxh6+ but after Kxc2 if he plays Kg7 it is met by Be5+ and black goes a rook down. His only alternative is to move the bishop again and I think that wherever it goes white has a winning reply.
Do you agree with this analysis? Have I found a position where promoting a pawn to a bishop is the best possible move?