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Let me define the problem clearly so as to avoid confusion. I am trying to find a position where:

  • The only move is a promotion move.
  • Said move needs to be promotion to a bishop. Not a queen, not a knight, and not a rook.
  • Where "only move" means either, a move that draws while all other moves lose, or, a move that wins while all other moves draw or lose.

I have been trying to come up with such a position for more than a week now without success. The following is my thought process.

AFAIK there are two main reasons for underpromoting:

  1. Underpromote to deliver a check/attack that a queen cannot fulfill
  2. Underpromote to avoid stalemate

The first scenario should be irrelevant because a queen has all the moves of a bishop.

For the second scenario, I thought about the fundamental cause of stalemate. Promoting to queen can draw, either due to its rook-move ability, or its bishop-move ability, or both abilities simultaneously.

Reason for stalemate Rook move Bishop move Both
Example FEN 8/k1P5/2P5/K7/8/8/8/8 8/6P1/8/8/8/8/8/k1K5 8/k1P5/8/2K5/8/8/8/8
Solution Promote to Bishop/Knight Promote to Rook/Knight Promote to Rook/Bishop/Knight

As you can see, there is no scenario where you have to get a bishop. Therefore I concluded that any singular reason alone cannot produce a position where promotion to bishop is the only move. In other words, there has to be other conditions in the position that prevent the choice of another piece, in particular that of the knight.

One idea that came to my mind is that you might need a bishop-move to prevent something, but this idea got shot down pretty quickly because if you need to prevent something, that means the opponent is threatening a move, which means there's no stalemate.


Currently, the only semi-plausible idea I have left is that promotion to knight leads to insufficient material, whereas promotion to bishop doesn't. However coming up with such a position is quite difficult as you can imagine (especially given the condition that it has to be the only move), so I have yet to think of one.

Edit:

Thanks to @bof for the solution along this line of thought! It wasn't too difficult after all - I simply overlooked the possibility of promoting with capture, which made it the only move.


I have asked this question to a few IMs and GMs on their streams to no avail (understandably so, since they still have to stream), so I am turning to StackExchange community for ideas and answers. Thank you in advance!

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    It is even possible that the only winning move is promoting to a second bishop of the same color! Not surprisingly, someone had the study idea before me. – Hauke Reddmann Feb 14 at 10:30
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    Another possible reason for promoting to bishop: to get stalemated because it's the only way to draw. – bof Feb 15 at 8:26
  • Addendum: THX to Siegfried Hornecker, he found (back) this: Alexey Troitzky, Shakhmatny Listok, 15. Januar 1925 [[FEN F8/p3B2p/1P5P/3P4/5r2/3k4/8/6K1]], 1.b7 Rg4+ 2.Kf2 Rg8 3.d6 Kc4 4.d7 Kb5 5.d8Q Rxd8 6.Bxd8 Ka6 7.b8B. It is fun to analyze what good the second bishop is for! – Hauke Reddmann Feb 15 at 9:35
  • @HaukeReddmann yeah I really enjoyed this one. Black's best attempt is to run to g8, and white is unable to force the king out of the corner, which means the h6 pawn will never promote. However white can occupy both a3-f8 and a1-h8 diagonals with the bishops, and have a Zuzwang trick when black eventually runs out of moves with their a pawn. Kf7# is a pretty cool move. – cyqsimon Feb 15 at 10:09
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There are numerous chess problems, whether a checkmating one or an endgame study, that feature certain promotions as a necessity to win. Stalemate is the long-known reason behind doing so. Here is a small sampling.

This problem is the first known one to use Bishop promotion, although it is a later correction. Even better it comes with double the fun!

[Titler "Ignazio Calvi  Le Palaméde 4/1836, White Wins (Correction)"]
[FEN "8/6PP/3Pn3/6b1/8/k3P3/p7/K2B4 w - - 0 1"]

1. g8=N! Bxe3 2. h8=B! (2. h8=Q? Nd4 3. Ne7 Nc2+ Bxc2 Bd4+ 5. Qxd4)    Nd4 3. Ne7 Bd2 4. Nd5 {And white wins!}

However, it can also be motivated by the "Dead Position" rule. In the below position, suppose White is down to seconds on the clock. They have no queens or rook nearby and stopping the clock to call in the arbiter might not help really. Thus, White must promote to a bishop, luckily just before the clock runs out. Now, the game is drawn and a half point is saved!

[FEN "8/3P4/8/8/8/7k/7b/7K w - - 0 1"]

1. d8=B
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    Oh wow damn. That first position is a really nice one. I couldn't see the idea until I put it through an engine and play it out! The double piece sacrifice to get the queen on d4 to self-stalemate was crazy! – cyqsimon Feb 14 at 0:58
  • Indeed it is! crazy! Watch out for more on the way! – Rewan Demontay Feb 14 at 1:02
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    Hi Rewan - yes this is the younger sibling of the dead position rule, whereby someone can’t lose on time if they can’t be checkmated. So White is still safe if he promotes to Q or R, as it’s impossible to be mated by bB - the white officer can always interpose or capture. However promotion to N would be a mistake. Q or R allows White to play on until the clock actually falls. Maybe can arrange pieces so that promotion to Q would be a stalemate, so R is the only one with upside? – Laska Feb 14 at 10:27
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    @Laska While you are correct, having Q/R promotions draw by stalemate really moots the point of my entire setup. – Rewan Demontay Feb 14 at 19:58
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    I don't understand the advantage of forcing the game into a dead position in the 2nd example. Black has insufficient mating material, so in the event of white's flag falling it would be a draw anyway, would it not? – Darren H Feb 15 at 10:13
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It's easy to come up with positions where promoting to knight leads to insufficient material but promoting to bishop doesn't. This one, for instance:

[FEN "N5q1/7P/8/8/8/7K/8/7k w - - 0 1"]
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    Ah nice one. I didn't consider the possibility of promotion with capture. This is a good way to make it the only move I guess. – cyqsimon Feb 15 at 9:12
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First I wonder what is the most economical presentation of bishop underpromotion.

[FEN "5n2/6Pk/4N3/7K/8/8/8/8 w - - 0 1"] 

White to play and win

It's mate in 6 (with a dual)

  1. gxf8=B! Kh8 2.Kh6(Kg6) Kg8 3. g6 Kh8 4. Kf7 Kh7 5. Sg5+ Kh8 6. Bg7#

Here's the only one in PDB or WinChloe with less than 6 pieces. Are knights more economical than bishops?

[FEN "2k5/8/2K1P3/b7/5B2/8/8/8 w - - 0 1"] 

White to play and win

Otto Dehler, after Luigi Centurini, Leipziger Tageblatt 1921 (reflected)

It's mate in 9

  1. e7! Bd8 2. e8=B! Bc7! 2. Bd7+! Kb8 3. Bxc7+ etc.

Let me show some other off-the-wall examples...

Here's one based on the 50-move rule:

[FEN "7N/pP6/k7/8/K7/8/8/8 w - - 0 1"]
  1. b8=B! White mates in 34.

On the other hand:

  1. b8=N? This is a cursed win. Checkmate could eventually be forced, as is often the case for KNNvKP, but a draw intervenes under the 50-move rule. Black can delay the first pawn move or capture until move 54.

Another example uses the obscure Dead Position rule and the clock. It doesn't use the half-dead rule, but it isn't disturbed by that. To force the bishop, I think we need to put Black under extreme time pressure (aka "Zeitnot"). The flag is trembling... :)

[FEN "6nk/7P/6pK/p5Pb/6p1/p5P1/P7/8 w - - 0 1"]

White to play & win over the board under FIDE rules, with Black under extreme Zeitnot.

Explanation: White can only promote. If 1.hxg8=Q/R+? then 1. ... Kxg8= stalemate. If 1. hxg8=N?, then Black might instead try 1. ... a4 with 2. Ne7,Nf6= stalemate for the other side! So in all these cases no checkmate is possible, and the game immediately ends in a draw before Black has to reply.

On the other hand, if 1. hxg8=B! then game does not end immediately, because at least in principle there is the alternative lifeline 1. ... a4 2. Bb3 axb3 3. a3,a4 etc, and there are checkmates around for either side. This is of course sub-optimal, Black should of course play 1. ... Kxg8=, but he has to execute this before his quavering flag falls.

Question is: would White bring the game into disrepute by playing like this? :)

OK here's a final one: the most economical of all, showing the half-dead position rule at work.

[FEN "2b4r/P7/8/8/8/8/8/4K3 w - - 0 1"]

White under Zeitnot to play & draw over the board under FIDE rules. No Black king!

So in the excitement of the game, bK has fallen off or been captured. By the half-dead rule, White can't win, because he can't checkmate, but he can play on for the draw. However, Black can checkmate White, so the position isn't fully dead. Checkmate does not require that the position be legal, but mere that the last move be legal. That's the FIDE Laws.

If 1.a8=Q/R/N, then a mate is possible with e.g. wKa1, wR/R/Nb1 bBa5, bRa8 & eventually Bc3#. Even though this mate may not be forced, the possibility of this is enough to keep the game alive. However, 1. a8=B makes the position dead, as neither side can now checkmate.

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    The B-promotion in "Otto Dehler, after Luigi Centurini" (a century old this year!) even shows up on occasion in analysis of Bishop endings from actual play. – Noam D. Elkies Feb 18 at 2:58
  • @Noam: apart from reflection, I suspect it's the only such KBPvKB position with P not yet on 7th rank. Or do you have others? – Laska Feb 18 at 5:11
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    It also works with the pawn on a few other files, and the Bishops elsewhere on the diagonal. In your diagram, bBc7 almost works, but besides 1 e7 Bd8! 2 e8B! White also wins with 1 Bg5, taking a few moves longer. Likewise it's tempting to retract 0 e5-e6+ Kb8-c8, but 0 Kd7 would also win two moves later. However, Kc5 Bh1 Pd5 / Ka8 Ba6 works: 1 Kb6 Bb7 2 d6! Kb8! 3 d7! Bc8 4 d8B! Bb7 5 Bc7+! Kc8 6 Bxb7+ and mates in 15, with each White move unique and each Black move offering the longest resistance (up to a tie with 4 . . . Ba6). – Noam D. Elkies Feb 19 at 0:24

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