# Construct your own retro #2

David113
Posers and Puzzles 13 Nov '06 06:12
1. 13 Nov '06 06:12
Find a position with 7 pieces or less, where no king is in check, it is not given who moved last, but it is still possible to conclude what was the exact position before the last ply.
2. 13 Nov '06 06:54
Originally posted by David113
Find a position with 7 pieces or less, where no king is in check, it is not given who moved last, but it is still possible to conclude what was the exact position before the last ply.
7 pieces with or without the kings?
3. 13 Nov '06 08:17
With.
4. 13 Nov '06 16:00
I think I found it after just 5 minutes. Makes me think it cannot be correct. So I PM'ed you my solution.
5. BigDoggProblem
Just...the Dogg
13 Nov '06 19:53
Originally posted by David113
Find a position with 7 pieces or less, where no king is in check, it is not given who moved last, but it is still possible to conclude what was the exact position before the last ply.
6. 13 Nov '06 19:58
That position cant be legal in my humble opinion. The only way for the King to get to a2 is via the back rank, but then the rook cant get to b1, since the last move must have been Kc1. Where is my thinking error?
7. 13 Nov '06 20:23
Originally posted by Darrie
That position cant be legal in my humble opinion. The only way for the King to get to a2 is via the back rank, but then the rook cant get to b1, since the last move must have been Kc1. Where is my thinking error?
I think the answer is correct!

The problem here is, that I cannot tell what the position before was. I only know the king moved from d8 to c8 but I don't know which piece it took there (or if he took a piece at all). This is covered in BigDoggs answer: There must have been a knight on c1, because that is the only piece that can get there in the move before White's last move. It could not have been unoccupied because then it really seems to be an illegal position as you pointed out.
8. 13 Nov '06 20:27
Ah I see!
Beautiful!
9. 13 Nov '06 22:404 edits
BDP's solution is not correct. Do you see why?
10. BigDoggProblem
Just...the Dogg
13 Nov '06 23:251 edit
Originally posted by David113
BDP's solution is not correct. Do you see why?
Edit: David113 cooked my first try by -1.Kd1-c1 Ka1-a2 -2.Rc1x~b1+ etc.

Good spot. I noticed that Pa3 is serving no useful purpose in my first try. I can make better use of that 7th unit to jam the a1 square, which (I believe!) saves my idea:

11. 13 Nov '06 23:31
Originally posted by BigDoggProblem
Good spot. I noticed that Pa3 is serving no useful purpose in my solution. I can make better use of that 7th unit to jam the a1 square, which (I believe!) saves my idea:

[fen]8/8/8/8/8/8/kPPP4/bRK5 w - - 0 1[/fen]
The bishop can only have got there through promotion, and the king cannot have moved to a2 afterwards so that position is impossible.

Does this work?

12. 13 Nov '06 23:342 edits
I think BDP's second solution is fine. The bishop promoted on a1. Then the black king came to a2 from b1. then the white rook came to b1, the white king came to d1 and the black knight came to c1. Anyway, if you don't like the black bishop on a1 you can replace it by a white knight.
13. 13 Nov '06 23:36
Originally posted by Ian68
the king cannot have moved to a2 afterwards so that position is impossible.
The king could still get to a2 through the first rank. I think BDP's solution is good now.
14. BigDoggProblem
Just...the Dogg
13 Nov '06 23:46
Originally posted by Ian68
Does this work?

[fen]8/8/8/8/8/8/kPPP4/b3RK2 w - - 0 1[/fen]
No, it does not; there are a ton of possible last moves, such as -1.R~(x)e1, -1.K~(x)f1, and even -1...Kb1(x)a2.
15. 13 Nov '06 23:49
Originally posted by BigDoggProblem
No, it does not; there are a ton of possible last moves, such as -1.R~(x)e1, -1.K~(x)f1, and even -1...Kb1(x)a2.
I can see yours works now, I didn't realise at first what the last move was supposed to have been.