Minor promotion or underpromotion is the promotion of a pawn to a knight, bishop or rook. The player that has moved the pawn to the promotion square has to decide which piece to select: a queen, a rook, a bishop or a knight. It can not remain a pawn and according to the FIDE rules of chess the choice becomes final as soon as the selected piece touches the promotion square.
In almost all games a pawn is promoted to a queen. Sometimes another piece is used, but most of the time this is done without a real need. The player just likes to win with a rook instead of a queen.
But in some cases a minor promotion is needed in order to win the game, most of the time to avoid a stalemate.
The following two diagrams show some easy examples. In the diagram on the left promotion of the pawn to a queen will result in a stalemate, but White can win the game by promoting to a rook (or by playing Kd6). In the diagram on the right promotion to a queen or a rook will result in a draw (stalemate), but promoting to a bishop will result in an easy win. In this case White may also win by playing Ke7.
8/k1P5/p7/P3K3/8/8/8/8 w – – 0 53
8/5P1k/5K1p/7P/8/8/8/8 w – – 0 51
Another interesting example is shown in the next diagram. Now White is forced to promote. He has to capture the rook. Promoting to a queen or a rook will result in stalemate, so the pawn has to be promoted to a bishop or a knight.
2r5/kP6/P7/K7/8/8/8/8 w – – 0 49
I should go for the bishop, but promotion to a knight will be ok.
49. bxc8=B will probably be followed by something like 49… Kb8 50. Bh3 Kc7 51. Kb5 Kd6 52. a7 Ke5 53. a8=Q Kf6 54. Qf8+ Kg5 55. Be6 Kg6 56. Qe7 Kh6 57. Qf6+ Kh5 58. Qg7 Kh4 59. Qg4#
A rather complicated example can be found in the next diagram taken from the game Sokolsky – Ravinsky. This example has been discussed by Tim Krabbé in his article about Practical Underpromotion.
4b1k1/P4pPp/1R3P1P/2r5/8/1P6/1K6/8 w – – 0 1
In this article he describes 47 examples of underpromotion in actual games. In the diagram on the left 66.a8=B is winning and it is White’s only winning move. The alternative moves are discussed in Tim Krabbé’s article.
Another interesting position that can be found in a number of actual games is the position given in the following example.
k6r/8/PP6/1KR5/8/8/8/8 w – – 0 49
49.Rc8+ Rxc8 50.b7+ Ka7 will result in a position that should be familiar by now.In this case 49.a7 is also winning
49…Kb7 50.Rc7+ Ka8 51.Ka6 1-0