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I was looking at minimal combinations of material needed to force checkmate (K+R, Q+B, R+N+N, etc). You end up with a sort of threshold of how much material is needed, and that sort of assigns a strength to the king. But that made me wonder if K+K is sufficient material to force checkmate. Obviously this is normally impossible, but in the hypothetical endgame where white has two kings and black has one (and make the obvious loosenings of the restrictions on moving into check), could the two kings force checkmate against the one king?

Alternatively, one could think of it as white having a king and a fairy chess piece that moves exactly like a king. Is this enough material to force checkmate?

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    This question doesn't make sense using the Laws of Chess. A king cannot check another king, because that would involve putting itself in check, which is illegal. Of course, it is also illegal for a player to have more than one king. Since you mention using a non-standard piece, you are introducing the idea of an undefined chess variant. If you would like to specify the rules for the chess variant, you might get some replies. At the moment, there is no reasonable way to answer this question.
    – jaxter
    Commented Jan 30, 2017 at 22:22
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    I'm not sure why this was closed. This is a totally sensible question in the context of Fairy Chess, which I've always thought was on-topic here.
    – Steven Stadnicki
    Commented Jan 31, 2017 at 20:37
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    Specifically: "Questions about variants may or may not be accepted, depending how far removed from the standard game it is. In general, a variant which uses the same board and pieces as standard chess is an on-topic subject." This is clearly a variant using the same board and pieces.
    – Steven Stadnicki
    Commented Jan 31, 2017 at 20:42
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    @StudentT I disagree emphatically - as I said, there are numerous fairy chess stipulations involving multiple kings. See, for instance, matplus.net/milanvel/FairyKings.pdf or jsbeasley.co.uk/encyc/175183.pdf .
    – Steven Stadnicki
    Commented Feb 1, 2017 at 5:50
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    @StudentT I agree with Steven. If the question was limited to the first paragraph, you would probably be right. But in the second paragraph the OP explicitly mention what could be a legal Fairy Chess piece (i.e. a piece that moves like a King), so I think the question should be reopened, maybe adding a "fairy chess" tag to it.
    – javatutorial
    Commented Feb 1, 2017 at 10:05

3 Answers 3

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If these are two real kings, it is not possible. This is because one requirement of checkmate is that the black king is in check which cannot be achieved with any of the white kings, since it would put the white king in check.

If you say that white has only one king and a nonstandard chess piece that moves exactly like a king (i.e. this piece can be put next to the enemy king), then yes, it seems possible and quite easy to mate the black king.

For example, put the king and “king” for on c3 and f3, and the black king on d5. As you can see the two kings almost cover the whole 4th rank. Pushing the black king to the side of the board and into the corner is quite easy, e.g. if black starts with 1... Ke5, 2. Kc4 Kd6 3. Kf4 Ke6 4. Kc5. Once you have the black king around the corner you put white's real king on c6 and give checkmate with the "king", the piece that moves like a king but really isn’t one, on g7.

Here is a demonstration of said strategy in the replayer. Remember, white’s standard king is on b2 and their other “king” is on f3!

[FEN "8/8/8/3k4/8/5K2/1K6/8 w - - 0 1"]

1. Kc3 Ke5 2. Kc4 Kd6 3. Kf4 Ke6 4. Kc5 Kd7 5. Kf5 Ke7 6. Kc6 Ke8 7. Kf6 Kd8 8. Kf7 Kc8 9. Ke8 Kb8 10. Kd8 Ka8 11. Kb6 Kb8 12. Kdc7+ Ka8 13. Kcb7#
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A King and a Man (or commoner; i.e., a non-royal King) can mate a lone King. The longest distance to mate on a standard 8x8 board is 18 moves.

The result quoted above was obtained by H.G. Muller in 2008, see this coment on chessvariants.com: http://www.chessvariants.com/index/listcomments.php?id=28770

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    Muller certainly got the result but I can't imagine that this wasn't already known earlier; I'd be a little surprised if the result (plus or minus a move or two) wasn't already known to T.R. Dawson or one of his compatriots in the early 20th century.
    – Steven Stadnicki
    Commented Feb 3, 2017 at 8:02
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    I just wanted to source my number, I didn't do deeper research in priority.
    – Sir Cornflakes
    Commented Dec 19, 2018 at 17:08
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Non-royal king is a combination of [0,1] leaper (wazir) and [1,1] leaper (fers) and in fairy chess called a erlking. About erlking, see my book Fairy chess endings on an n x n chessboard (2017), page 545.

The ending of king + erlking against bare king on board 8x8 is won for the stronger side and the longest win has 18 moves. Also the ending of two erlkings against king is a general win and the longest win has 16 moves.

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