Puzzle: mate in two, and the queen is the wrong promotion
Published · By TG, the site operator
Today's note is a puzzle: White to move and mate in two. There are only five pieces on the board:
- White: king f4, rook g8, pawn e7
- Black: king h6, bishop f8
If you use an analysis board, the position in FEN is 5bR1/4P3/7k/8/5K2/8/8/8 w - - 0 1.
Before you open the answer
The pawn on e7 has two ways to reach the last rank. It can step straight to e8, or it can take the bishop on f8, because a pawn captures one square diagonally forward and a pawn that lands on the far rank by capturing is promoted just the same. Either way White then picks a queen, rook, bishop or knight.
The natural choice is to take the bishop and make a queen, which even comes with check. Work out why that is not enough, then look for a quieter promotion that leaves the black king exactly one square to go to. A habit worth keeping: before you promote, name the squares around the enemy king and ask which of them each of the four pieces would cover from its new square.
Show the solution
1.exf8=N! The pawn takes the bishop and becomes a knight. It does not give check, but from f8 the knight guards g6 and h7. The rook on g8 already controls the g-file and the white king guards g5, so h5 is the only free square, and 1...Kh5 is Black's only legal move.
2.Rh8# The rook moves onto the h-file. The black king cannot stay on that file, the white king still covers g4 and g5, and the knight covers g6, so there is no way out. No other second move mates.
Why not a queen? After 1.exf8=Q+ Black replies 1...Kh5. A queen on f8 reaches g7 and h6 but not g6 or h7, so after 2.Rh8+ the king simply steps to g6. White is still a queen and a rook up, but there is no mate on move two. A rook or a bishop on f8 fails too, because the king escapes to h7 or h5, and 1.e8=Q without the capture is met by 1...Bg7 or 1...Bd6+.
How the answer was checked
The solution was checked by computer with the python-chess library. The script tried all 24 legal first moves for White. None of them mates at once, and 1.exf8=N is the only one after which every Black reply can be met by checkmate. After 1.exf8=N Kh5 the only mating move is 2.Rh8.
The same choice on this site
You will meet this decision in your own games here. Whenever one of your pawns reaches the last rank, by a straight move or by a capture, the board stops and opens a small window with four buttons: Queen, Rook, Bishop and Knight. The move only goes ahead once you have picked one, so take a second to look at the king before you click. The move list records your choice in standard notation, so this solution would appear there as exf8=N followed by Rh8#, and the game status then reads White wins by checkmate. Our rules guide covers promotion and the pawn's diagonal capture, and the glossary explains the word underpromotion.
Sources
- Chess rules: the complete guide for this board — chessplayonline.com
- FIDE Laws of Chess taking effect from 1 January 2023 — FIDE
- python-chess: a chess library for Python — python-chess
Written by the operator of this site from the sources listed above. Spotted a mistake? Email [email protected] and it will be corrected.