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Find 2 chess positions in which the only move is a promotion to a knight, or bishop (1 position for each piece).

However the goal of the underpromotion must NOT be one of the following :

  • Preventing a stalemate
  • Achieving checkmate (or achieving checkmate faster than without underpromotion)

Additional rules :

  • The promoting side may not lose the game with perfect play from both sides
  • Insufficient material / 3-fold repetition / 50-move rule draws cannot happen (In order to prevent trivial king-queen forks)

Can you figure it out? The positions do not have to be reachable in a legal chess game.

Bonus questions (I haven't found an answer to these questions, so they are not required to be marked as accepted):

1) Find a position following the same principle, but with a rook promotion instead.

2) Find legal positions following the same principle with knight/bishop/rook promotion.

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  • $\begingroup$ Must the chess position be reachable (that's what you mean by "legal" in the bonus question, or is it something else, like not having two kings of the same color)? Some of the solutions have positions which are impossible to achieve in a legal game (like one side has more pawns on the same file than the other has pieces missing ) $\endgroup$ – vsz Apr 7 at 11:02
  • $\begingroup$ A legal position is not required. Bonus question 2 forces you to have a legal position, but the main problem doesn't. Of course you can't have two white kings - so 'legal' does mean 'reachable' here. $\endgroup$ – classicalMpk Apr 7 at 11:03
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Being a chess composer myself, one possible motivation came to my mind right away: self-stalemate. White cannot win, so they must secure a draw via underpromotion. While there are plenty of problems that show it, here are three problems where the rook, bishop, and knight promotions are legally maximized.

I will leave to you to figure out which is which, so you may try to solve it own your own! Solutions are hidden if you need them. All three problems have the same stipulation: It's White to move and draw.

  1. Grigory Popv, SuperProblem.com 9/24/2017

    enter image description here

Solution:

1. c8=R! Rdcx8 2. dxc8=R Rxc8 3. bxc8=R h1=Q 4. Nc3+ bxc3 5. Rc4+ b4 Stalemate

  1. Mikhail Zinar & Sergiy Diduhk, EG 2009, 1st Prize

    chess position

Solution:

1. d8=N! Rxd8 2. exd8=N! Rxd8 3 cxd8=N Nc6 4 e7 Nxd8 5. exd8=N Kg3 6. e6 Bxb6 7. e7 Bxd8 8. exd8=N Kh3 9. d7 Kg3 10. Nc6 Bxb7 11. d8=N Ba8 12. Nb7 Bxb7 Stalemate

  1. Mikhail Zinar, EG-50 2016, 1st Prize

    chess position

Solution:

1. h8=B! Rxh8 2. gxh8=B! Nd2 3. g7 Rh3 4. g6 Rxh8 5. gxh8=B! Nb3 6. g7 Nd5 Stalemate

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  • 1
    $\begingroup$ Amazing ! self-stalemate is exactly what I had in mind. My solutions were way less elegant : they involved a king on g7 and a bishop prom. on h8, and a similar pattern for knight. These compositions are way better : I think you deserve to be the accepted answer. $\endgroup$ – classicalMpk Apr 7 at 9:30
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We can achieve the stated goals if

the underpromotion secures a draw (and other moves lose), rather than securing a win.

Knight:

8/1q1P1k2/8/K7/8/8/4b3/8 w - - 0 1
enter image description here
d8=N+ is an easy draw; promoting to anything other than a knight allows ... Qb5#; not promoting allows black (e.g.) to take the pawn and then deliver checkmate at leisure.
Obviously this is a legally achievable position.

Bishop (this one was harder):

K6q/PP4r1/b7/n7/8/8/7k/8 w - - 0 1
enter image description here
After b8=B+, if black captures then white recaptures, promoting to a queen, and should be able to draw easily; if black doesn't capture then white is stalemated. After a promotion that doesn't give check, Bb7 is mate. After promoting to a queen, black moves the king and then mates a few moves later even though white gets to capture the BQ.
I'm fairly sure this position is reachable with legal (but insane) play. There aren't any obstacles in getting backwards from it to a position where the WK is no longer boxed into a corner, and I can't see any reason why that shouldn't then be reachable. But I haven't actually tried to build a game that does it.

Doing a similar trick with a rook promotion is harder because

you can't arrange for the newly promoted rook to be pinned. Or rather, you can but it will be able to capture the pinning piece, and a queen would be able to do the same, and it's then hard to see how having a rook could be better than having a queen.

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  • $\begingroup$ Ah, yes - the fork ! Your bishop answer is amazing. Regarding the knight answer, it is not the intended one, but it fits the rules. I think it won't be a big deal if I add one or two rules to prevent this trivial fork (and the knight solution I have in mind is pretty interesting!). $\endgroup$ – classicalMpk Apr 7 at 0:58
  • $\begingroup$ It feels as if it wouldn't be difficult to make a knight one on almost the exact same principles as the bishop one. $\endgroup$ – Gareth McCaughan Apr 7 at 1:03
  • $\begingroup$ (I had a feeling the knight solution wasn't the sort of thing you had in mind! But it does meet all the criteria you stated.) $\endgroup$ – Gareth McCaughan Apr 7 at 1:06
  • $\begingroup$ Yes. I found my knight and bishop solution with a totally different approach : the bishop one is easier than the knight and can be reproduced in ~5 minutes if we know the goal of the underpromotion. The knight solution works the same way, but it is harder to achieve. The rook one is probably doable too but I haven't found it yet. $\endgroup$ – classicalMpk Apr 7 at 1:27
  • $\begingroup$ @GarethMcCaughan Jtlyk, you’re idea for a drawing bishop promotion has already been found by Tim Krabbe: timkr.home.xs4all.nl/chess2/minor.htm $\endgroup$ – Rewan Demontay Apr 7 at 15:44
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Rook

White to move:

FEN: 1BRBRQBK/P1P1PPPP/Ppp1p3/3bp3/4p3/8/8/4n2k w - - 0 1 chess position

Explanation:

Black threatens to checkmate in three knight moves, e.g., Ng2, Nh4, Ng6#. White's only legal move is a8. White cannot do anything to directly stop the checkmate, only delay it one move by 1. a8=Q and 2. Qxc6 Bxc6. However, white threatens to stalemate after two moves by 1. a8=R and 2. a7. Black cannot do anything to prevent this, as the bishop and knight need more than two moves to allow white to move. Therefore, the rook underpromotion is the only move that draws instead of losing.

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