Chess Wiki

Underpromotion is where the pawn reaches the 8th rank and promotes unexpectedly. It is similar to promotion, but underpromotion turns the pawn into Rook, Bishop, or Knight, instead of a Queen. Underpromotion is sometimes better than promoting to a Queen because in some situations, promotion to a Queen results in a stalemate. Here are some circumstances where underpromoting is necessary: 

  • The Knight has its own unique movement. No other chess pieces has the ability to move like a Knight. 
  • Promoting to a queen allows a stalemating combination.
a b c d e f g h
8 a8 b8 c8 d8 e8 f8 g8 h8 8
7 a7 b7 c7 d7 e7 f7 g7 h7 7
6 a6 b6 K c6 P d6 e6 f6 g6 h6 6
5 a5 b5 c5 d5 r e5 f5 g5 h5 5
4 a4 b4 c4 d4 e4 f4 g4 h4 4
3 a3 b3 c3 d3 e3 f3 g3 h3 3
2 a2 b2 c2 d2 e2 f2 g2 h2 2
1 a1 k b1 c1 d1 e1 f1 g1 h1 1
a b c d e f g h
Saavedra position

Here are a sample of chess moves in the game Saavedra, Fernando vs. Unknown player. 

/*99 times out of 100, such a position would be totally drawn. But the terribly awkward position of Black's rook gives White a golden opportunity. 1. c7 Had Black's rook been anywhere but d5, a move to the c-file or 8th rank would secure an immediate draw. As it stands, the rook must resort to a series of desperado checks. 1... Rd6+ 2. Kb5 The king must tread carefully. Both*/

(2. Kb7?? Rd7 and)

(2. Kc5?? Rd1 are to be avoided at all costs.)

2... Rd5+ 3. Kb4 The cat-and-mouse game continues. 3... Rd4+ 4. Kb3

(4. Kc3 leads to a transposition after 4... Rd1 5. Kc2 Rd4)

4.../* Rd3+ 5. Kc2 Black has finally run out of checks. High time, it would seem, to stop the clock and head to dinner. 5... Rd4! Not so fast! A keen tactical eye will quickly notice that 6.c8=Q allows 6...Rc4+!! 7.Qxc4, when the Black monarch is stalemated. Indeed, this position was thought to be drawn until Mr. Saavedra came around. In general terms, Black's king is abysmally situated, but after 6.c8=Q this lack of space is precisely what enables him to escape with a draw. There is seemingly no way around this irksome tactical quirk, unless of course... 6. c8=R!! Unless of course White doesn't promote to a queen! This incredible move, the quintessential example of underpromotion, has been published, analyzed, re-published, and re-analyzed by chess players and enthusiasts all over the world. The rook's inferiority works in White's favor, as 6...Rc4+ obviously loses its meaning. Black is therefore obliged to resort to passive defense.*/

(6. c8=Q?? Rc4+! 7. Qxc4)

6... /*Ra4 But now, with both of Black's pieces so vulnerable, it is no surprise that White has a deadly tactical shot at his disposal. 7. Kb3! Game over. There is no way to simultaneously defend the rook and parry the mate threat. 1-0*/