Illegal Position Contest!

The position at #392 has a black queen at c6.

Here is my approximation of an illegality proof.

There have been no promotions and no pawns or knights have been captured. Black's two captures were of a white rook at d6 and the f1 bishop at a6. Before any captures occur, neither side can release a piece to be the first piece captured, so the first capture was of the c8 bishop at home. Then after Pc7-c5, the a8 rook can escape to be captured at f3, allowing the h1 rook to escape and be captured at d6, allowing the h8 rook to escape and be captured at c3, releasing all the remaining white pieces. For the white king to reach h8, the b7 pawn must move to a5 after capturing the f1 bishop on a6. After the king-side pieces are in place, the only path for a black knight to reach b7 is from d8, after which the black king can move from c7 back to d8. But then there is no way for the white queen to reach b8, where it could not have already been without checking the black king.

Good first attempt! However, why cannot the white king enter via g6? The king can sneak past before the pieces on the NE side get locked, allowing bNb7 to come from a5.

Good question. The king can come that way. But after the queen moves to b8 and the knight to b7, White has one more move with the pawn at c3, while Black needs two moves to get the queen to c6 and the pawn to a5.

Your original position with the missing black queen was legal.

And one more tempo for the bNb7. In the case of the missing queen, the last move can be Nxc8.

A tempo isn't needed for bNb7 after the knight is already there.

Or Pa6-a5.

Oh, you were thinking of white to move in that position. My bad. Also, the try h7-h6 to free the wNg8 means the wK cannot enter via g6.

Or c6-c5, as well as h7-h6.

It's sad that cobra91 is no longer around, since someone has finally studied the position.

Yeah, he (or she) has created challenging and excellent illegal positions in the past. Another example is #289, where an illegality proof was already given at #394.

Either I'm dumb or the pawn on f3 can't get there.

nvm im dumb

Ok this is my third time writing the illegal proof because of the dumb site: Basically you can't get the knight to e1 without sacrificing the h1 rook's ability to go to the e-file which is necessary for the black h-pawn to go to e2 and then promote to the f1 rook in the position because it needs 3 captures while without the h1 rook there is only the c1 bishop and the a1 rook.

It seems clear that the h8 rook was captured at f3, and the h7 pawn captured a white bishop or rook on f1. So all four of Black's captures were by the h7 pawn of White's rooks and bishops on the way to f1. Its first capture couldn't be on g6, since no white bishop or rook could get there before the h8 rook was released. The pawn's first capture must have been on g5, after White played d3 releasing the c1 bishop to be captured. The black pawn's only light square capture is at f1, so it must capture the white rooks at f4 and e3. The f1 bishop and h1 rook are trapped until the capture on f3. After that capture, a white knight can't move to e1, so it must have already been there before the capture. But after the capture on f3, the knight on e1 will never move again, keeping the h1 rook from getting out to be captured.

What I said in a longer version.

That's true. daStrwbrry likes to see the details.

I think both explanations did well, but kracker12345’s explanation forgot to consider that the h1 rook could escape after exf3, when the knight can get to e1 via d3 (proved illegal by n9531l1).

If the wPh3 is moved at h2 and the wK is anywhere, is the position still illegal?