Illegal Position Contest!

It would be nice if you could find a position whose illegality is less obvious.

Black's only capture is of the a1 rook, so the h7 pawn has to promote without leaving the h-file. It and the rook at h2 have to both be in place before the white gh pawns are in place, and then there's no way for the white king and rook to trade places.

With the white king and rook swapped, it's easily legal.

here are 2 positions i think are illegal but do not know

The Easiest Illegal Position to make

Proving that a position with 22 black pawns isn't interesting at all

This is probably the best illegal position puzzle on the thread. This position is just pure genius.

In the second position, for minimum (14) captures, black’s gh pawns need to capture once each (i.e. 2 captures total) to wrap around white’s gh pawns. Then 4 captures are required for ef pawns to wrap around each other: two white and two black. Same logic for cd pawns, and for ab pawns.

Therefore white has captured 6 times and black 8 times. But white only has 7 pieces to feed black, so black is one short of captures needed.

For the first position at #8335, black captured a piece on b-file, f1 bishop at home and another piece. No cross-captures happened, so bPe6 comes from e7, and bPd6 from d7. White promoted to a bishop, and black is missing 4 pieces. White’s pawns can capture just twice, but it requires cxd7 and fxe7 (then e8=B, the right square colour for wBa6). Otherwise white captures four times (due to parity of pawns switching files).

However, the problem is with releasing the top left cluster. wK can’t get to c5 as bPd6 can’t retract check. wK can’t retract to d5 as that forces bPe6 to go back to e7, but bBa5 has no way to get there if it was stuck on f8. Also white rooks/queen can’t retract to b7 because of checks. There is a partial solution though: get wB to b7 and retract a6xNb5, which releases some pieces.

Now the last move Qc8-b7# either came with a capture or not. There are 2 cases: [1] or [2].

Case [1]: last move Qxb7#

Problem is now the kings, queen and promoted bishop are still stuck due to illegal checks. If wK retracts to b5, the checking bPa6 must capture the dark squared bishop to retract the check, but a6 is a light square. Again wK can’t retract to c5 because bPd6 can’t retract check. If wK retracts to d5, bPe6 must go back to e7, meaning that white can’t play fxe7, so white must capture all 4 pieces with pawns. But along with the piece captured on b7, white captured 5 times but black only has 4 to give. Hence [1] is not possible.

Case [2]: last move Qb7#

This forces black’s last move as h7-h6. If the wN captured a black piece on b5, this would transpose back into [1], which we know is impossible. Otherwise, the cluster still needs to be released. The wK can’t retract to c5 (as I mentioned before) so it retracts from d5. This again forces bPe6 back to e7, but again white can’t play fxe7, so white must capture 4 pieces. But bBa5 must go back home at f8, and unmoved bBf8, bPg7 and bPh7 trap the h8 rook and prevent it from being captured by white cf pawns. This means that the rook cannot contribute to white’s captures, so [2] is not possible either.

Since neither [1] nor [2] is possible, the position is illegal.

TL;DR - No legal way to reach that configuration of pieces in NW corner.

How did the d8 bishop get there in the first place

It looks so illegal that it's surprising how much work it takes to prove it.

cxb3

cxb3 forces 4 white captures, so Qb7# did not come with a capture. It allows black tempo moves to get back to the starting position of [1]. There is also an alternative to releasing that position, which is unpromoting the white bishop on a8:

But now the b-pawns can’t get past each other without a capture. Also, the black king rook can’t go back home on h8.

I think this is illegal. Will this be proven wrong?

This might be proven wrong.

ooh...

Positions with promoted pieces and 20 pawns on the board, or extra kings, or both sides in check are not interesting. I think you can do better than this.

ye

I'm curious about this one:

Easily legal. The only challenge is if you have to triangulate to get a certain side on move.