underpromotion

In chess, "underpromotion" refers to the act of promoting a pawn to a piece other than a queen upon reaching the eighth rank--that is, promoting to a knight, bishop or rook. One of the most frequent reasons for doing this is to avoid stalemating the opponent, but there are many other plausible situations where underpromotion occurs. Underpromotion to a knight, for example, can be deadly if the newborn knight immediately forks the opponent's king and queen from its promotion square. Underpromotion can even be a psychological tactic, to emphasise to the opponent that a queen is not needed to defeat him, or (in cases where the promotion square is under attack) to make him unsure whether to capture the promoted piece.

For one of the most famous endgame positions in which underpromotion plays a vital role, see Saavedra position.

The Pawn in chess has long been one of the most studied pieces in the game. François Philidor famously declared "Pawns are the soul of chess", and the defense which bears his name creates a tense, waiting-game for the black pieces as its motif is to watch the center, evaluate pawn structures, and attack the weak points therein. Pawns, while being the most numerous piece on the board, also have more "special moves" than any other piece in the game. Sure, the King might be able to castle with a Rook, but the lowly pawn has three tricks at its disposal: the first move double step, the ability to capture en passant, and the ability to promote itself to a new piece upon reaching the 8th rank on the board.

Historically, the pawn used to have no selection power upon reaching the 8th rank. In chaturanga, the India-developed variant and almost-certain precursor to modern chess, the pawn would automatically promote to the Vizier - chess' "proto-Queen". A weaker version of the modern piece, the Vizier could move only one square along either diagonal from its starting position¹. In chess, the pawn may promote to any piece other than a King or another Pawn, a feat that happens during approximately 1.5% of games. When a promotion does occur, the promoting player chooses a Queen nearly 97% of the time. A knight is selected 1.8% of the time, a rook 1.1%, and the poor bishop only 0.2% of all promotions². These three piece selections are referred to, in aggregate, as underpromotions.

There are three valid reasons why a player might choose underpromotion.

1. When choosing a Queen would lead to an immediate stalemate
2. When choosing a Knight would lead to an immediate gain of material via a fork
3. When choosing to illustrate to your 8 year old nephew that, yes really, I did lose that last game on purpose and, yes really, I am promoting a fifth bishop, Timothy!

The first two reasons are illustrated below, with brief annotations.

Scenario 1:
Black to move, White wins.
White to move, brilliant underpromotion.

	A	B	C	D	E	F	G	H
8									8
7						♙		♚	7
6								♙	6
5								♔	5
4									4
3									3
2									2
1									1
	A	B	C	D	E	F	G	H

Underpromotion Scenario 1: Black to move

Move	White	Black	Annotation
1	f7	Kh8	Forced
2	f8=Q+	Kh7	Forced
3	Qg7#	1-0	Very easy mating pattern

Underpromotion Scenario 1: White to move

Move	White	Black	Annotation
1	f8=B!	Kg8 (or Kh8)	f8Q? would have created an instant stalemate, with no free squares for the black king. Bishop avoids stalemate, and controls the queening square for the remaining rook pawn!
2	Bg7	Kh7	White connects pawn and bishop, Black hopes to wedge himself on h7
3	Bf6	Kg8	White owns h8, Black forced off its mark
4	Kg6	Kf8	White takes the opposition, Black's move is forced
5	h7	Ke8	White has too much synergy, Black's move is forced
6	h8=Q+	Kd7	White achieves a completely overpowering endgame

Further study of Scenario 1 is documented within Saavedra position.

Scenario 2:
Jones, Gawain C (2664) vs. Raznikov, Danny (2492)³
PokerStars Open | Douglas | Round 9 | 12 Oct 2014 | ECO: B90 | 1-0

	A	B	C	D	E	F	G	H
8									8
7		♟	♖	♙					7
6	♟								6
5		♚	♙			♟			5
4		♟							4
3					♔			♜	3
2	♙								2
1									1
	A	B	C	D	E	F	G	H

Underpromotion Scenario 2: White to move

Move	White	Black	Annotation
1	Kd4	Rh1	Black determines its most efficient defense of d8 will be via x-ray attack and delivering check to the King on d4. White has other plans.
2	Rb7+	Kc6	White complicates Black's game precipitously. The King only has three available squares to escape the Rook's check. Two of those options surrender a tempo, and thereby surrender d8.
3	d8=N!#	1-0	A wonderful underpromotion to win the game!

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Sources
1 - Wikipedia. "History of chess". Accessed 11/15/2018.
2 - Wikipedia. "Promotion (chess)". Accessed 11/15/2018.
3 - Chess.com. "A guide to underpromotion". Accessed 11/15/2018.

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