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The post "A mysterious mate in one" made me think of one of my last problems which is also a Mate-in-One with no black King:

    [Title "White to move and mate in one"]
    [FEN "1B1NB2K/P2RN3/8/1R6/1P5P/4P2Q/1PP2PP1/8 w KQkq - 0 1"]

All the white pieces are there but none of the black ones.
White (at the bottom) to move.
The position is legal, except that the Black King (and the Black King only) is not displayed. Find it - there is but one place.
Mate in one.

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    Hi Xavier...how confident are you about this? There are quite a number of safe squares; I count 14 in total. There doesn't appear a single move that results in all the square being covered. I might just being missing something conceptually – David Bateman Nov 18 '20 at 0:21
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    I made a mistake in the position. Fixed now, sorry... thank you for your comment – Xavier Labouze Nov 18 '20 at 12:27
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    I don't understand the task. Is the idea that we are supposed to find a configuration of the black pieces such there is precisely one mating move for White? But there still seem to be several solutions which could have arisen from a game (with a different White mating move). For example, black king on a8, black knight on a1, unique mate Qf3#. Or black king on h6, black pawn on g6, black knight on f6, unique mate Nf7#. – James Martin Nov 18 '20 at 17:32
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    @JamesMartin- The idea is to find the only place where the black king can be - in a legal play. Then Mate it in 1. – Xavier Labouze Nov 18 '20 at 18:00
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    @XavierLabouze Nice puzzle! – James Martin Nov 18 '20 at 21:23
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Your post is perfectly fine here, my friend. Chess problems and puzzles are on-topic here, and we do need more. All kinds of all sorts are welcome. We just need to bring them here!

I think that out of the 14 open squares on the board, only:

placing the Black king on both a6 works!

This is because placement on the squares:

a1, a2, a8, a4, b1, c1, c4, d1, d2, e1, e2, f1, f6, f8, g1, and h6 all result in an illegal position. This is due to the fact that Black either has no last legal move if it is White's turn, or it would've been impossible for the Black king to get around the current White pawn structure. Thus, they are not options at all. This explains the "retrograde-analysis" tag.

Therefore, remaining the square works:

with the solution checkmate being 1. Rd6#.

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    Thank you for your answer. I made a mistake in the position, fixed now. – Xavier Labouze Nov 18 '20 at 12:26
  • Thanks Rewan - really appreciate :) – Laska Nov 25 '20 at 17:25

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