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Is it unsportsmanlike to underpromote a pawn to improve my chances? Sometimes I reach a won endgame where underpromoting my pawn will result in a slower but easier final checkmate.

Old example, easiest to understand: I was far better practiced at K-R vs K than K-Q vs K so I promoted a lone pawn to a rook. Improvising K-Q vs K would have had too high of stalemate chance for my taste.

Recent example: I deliberately underpromoted a pawn to a rook rather than a queen while having a queen. This makes mate in 9 moves worst case rather than 8 and again reduces the accidental stalemate odds dramatically.

I barely care about my opponent not resigning when he should. Primary reason: I once played out a losing game, sacrificing my last piece to push the ending int B-N endgame. My opponent said I should resign as he had a book endgame. I said, "Yes, but I don't think you can do it." Fifty move rule got him: B-N is tight and he was out of practice.

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    No, it is not unsportsmanlike.
    – Cleveland
    Jul 1, 2018 at 20:35
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    "I barely care about my opponent not resigning when he should" -- if you really don't care you would not mention it at all. Now you sound like a TV drama character, "I don't care you don't love me anymore!" (tear droppin' =)
    – lenik
    Jul 2, 2018 at 10:19
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    @lenik: Some people would call failure to resign a lost cause unsportsmanlike and I wanted to discourage answers depending on it.
    – Joshua
    Jul 2, 2018 at 14:05
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    hahaha love the ending. But in the time you came here to write this you could easily have learned the KQ vs K ending. You shouldn't be too afraid of stalemates, and put your time on learning properly the basic endings. That will save you a lot of points since you will be able to mate faster, avoiding losses/draws on time, and with practice it should be fairly easy to avoid stalemates. For example, in KQ vs K you can always mate in 10 or less moves, in KR vs K you can always mate in 17 or less moves. That could be the difference between winning and losing.
    – Ivan Lerner
    Jul 12, 2018 at 22:21
  • @IvanLerner: I said old example. It does not represent my current skill level.
    – Joshua
    Jul 12, 2018 at 23:29

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If, as you say, you are "improving your chances", then it's not unsportsmanlike. Even if your opponent misunderstands, he doesn't really have much to complain about - if he doesn't want to play on he is free to resign.

I barely care about my opponent not resigning when he should.

Good, because demanding that your opponent resign IS unsportsmanlike.

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It's ok, top players do it too. But be prepared to face the consequence when the underpromotion doesn't work out.

http://www.chessgames.com/perl/chessgame?gid=1879525

(Move 77... f1=N by Nakamura)

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    Nice blunder that one.
    – Joshua
    Jul 2, 2018 at 14:03
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    Hah, I saw that one live. Incredible.
    – BlindKungFuMaster
    Jul 5, 2018 at 13:42
  • Even worse! A bishop would have done it too, f1N is technically no blunder, run the game through a tablebase and repeatedly hit your head against the wall...Admittedly blitz, but super GMs should know their rook endgames by heart!
    – Hauke Reddmann
    Jul 28, 2022 at 7:36
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You may choose any legal move you like; your opponent's sole recourse is to prove that it wasn't good enough.

This also applies to fighting on in lost games. There is always the possibility that the opponent will blunder away their advantage, or even the game itself.

I applied this principle when faced with an opponent with a bare king who refused to resign. I promoted my two pawns to knights for a total of three, and delivered a novel checkmate with K+3N. (The theory is known, apparently but I couldn't find any other examples so I developed my own).

It was a lot more challenging than just ginning up a simple Q+R mate, and even drew a small audience.

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Underpromotion might be "unsportsmanlike" if you do it toy with your opponent (e.g. promoting several pawns to Bishops for the heck of it), but if you do it to make the win easier then there's absolutely nothing wrong with it, even if it's because you're more comfortable winning with KR/K than KQ/K (which must be very unusual, but is not a failure of sportsmanship).

Tim Krabbé has a nice collection of examples of promotion to R or B in actual play: https://timkr.home.xs4all.nl/chess2/minor.htm . In a few cases the underpromotion is the only winning move, or at least the only winning promotion (because Q would stalemate and N is not strong enough). But there are also cases where a Queen would win too but with more difficulty because you must deal with a stalemate defense -- close to what you describe albeit at a higher level of play. See the four examples introduced by

in all of the following examples promotion to Queen would also have worked. But sometimes it would have been tricky, and in most cases the Rook promotion was the most sensible and practical thing to do: why think if you don't have to.

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Here's a game with pawn promotion to a Knight for checkmate. Nothing unsportsmanlike when one plays to win as far as I am concerned.

Zvonimir Mestrovic vs Svetozar Gligoric

https://lichess.org/RiMAvMwD

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