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I assume most people on PSE know a bit of chess

With white to move, this is checkmate (1. Qb7#):

Checkmate

That's boring. Let's try something more interesting. With black to move, black can FORCE white to checkmate (1... Qxb7+ 2. Qxb7#):

Force checkmate

We can go another step. In this position, white can force black to force white to checkmate (1. Qxb7+ Qxb7+ 2. Qxb7#):

Force forcing checkmate

I'm going to call this a forced checkmate in 2 (an abuse of traditional problems where you have to force checkmate in 2).

I can do forced checkmate in 12 (1... Qxb7+ 2. Qxb7+ Qxb7+ 3. Qxb7+ Qxb7+ 4. Qxb7+ Qxb7+ 5. Qxb7+ Qxb7+ 6. Qxb7+ Qxb7+ 7. Qxb7#):

Forced checkmate in 12

Can you do better?

NOTES:

  • There must be exactly one line after the first move, otherwise it is not strictly forced and I have to think too much.
  • The position must be theoretically possible (so not 10 queens of one colour, 11 bishops of one colour, etc.)
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    $\begingroup$ @humn you can just put s (small), m (medium), l (large and default), or h (huge) after the 5 letter imgur address. m works best for me $\endgroup$ – Beastly Gerbil Mar 9 '17 at 21:59
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    $\begingroup$ related, cross-site dupe: chess.stackexchange.com/q/8789/9025 $\endgroup$ – Herb Mar 10 '17 at 2:38
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    $\begingroup$ @ais523 But selfmates can have multiple branches. Also, selfmates would only be forced checkmate in (odd #). $\endgroup$ – boboquack Mar 10 '17 at 23:21
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    $\begingroup$ I have a question - why not 2 ... Qbxb7? $\endgroup$ – Wen1now Mar 11 '17 at 7:31
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    $\begingroup$ I’m voting to close this question because open-ended puzzles are off-topic as of May 2019 $\endgroup$ – bobble Jul 21 at 18:46
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Yes, I can do better:

I can do forced checkmate in 13 (1... Qbxb7+ 2. Qxb7+ Qxb7+ 3. Qxb7+ Qxb7+ 4. Qxb7+ Qxb7+ 5. Qxb7+ Qxb7+ 6. Qxb7+ Qxb7+ 7. Qxb7 Qxb7#): enter image description here

(sorry for stealing your answer)

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Here is a new record of 16 moves that is a slight modification of a position made by Bernd Schwarzkopf and Karl Scherer in Feensnach 1980 on page 13. A PDF can be found here.

enter image description here

FEN: QKN4r/nRRRRRRr/kq6/p1q1R3/1P1q1b2/4q3/5q2/1r4q1 w KQkq - 0 1

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In this position that I composed long ago (this position is legal), every move is completely forced and it is checkmate in

!8 moves (15 ply): 1. Qxc7+ Qxc7+ 2. Qxc7+ Qxc7+ 3. Qxc7+ Qxc7+ 4. Bxc7+ Kb7+ 5. Bb8+ Qxe7+ 6. Qxe7+ Qxe7+ 7. Qxe7+ Qxe7+ 8. Rxe7#

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  • $\begingroup$ I alrighty have you beat: sorry! $\endgroup$ – Rewan Demontay Jun 25 '19 at 22:46
  • $\begingroup$ And this question is about a selfmate:You could add in the black queen moving to her square for 16 like mine got a tie. $\endgroup$ – Rewan Demontay Jun 25 '19 at 22:48
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    $\begingroup$ Also, funnily enough, before I even knew you were on SE just now, I cited your chess com forum post with this position in this CSE question, =D: chess.stackexchange.com/q/8789/15543 $\endgroup$ – Rewan Demontay Jun 25 '19 at 22:51
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    $\begingroup$ Hah that's really interesting. Yes I could place the black queen on b7 for a selfmate but I just posted the position as originally intended (all moves forced). It was really cool to see the 16 ply forced checkmate, maybe I'll try to tie that one day :) $\endgroup$ – shoopi Jun 26 '19 at 9:19

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