Illegal Position Contest!

You shouldn't have to think very long about that. But what do you think about this one?

If you care about making positions that people find interesting, don't put pawns on the 1st or 8th rank.

Yahoo I made an illegal position that isn't extremely obvious

Yes, you can be excited about "somewhat obvious". Is the one I posted legal?

Legal with White to play, illegal with Black.

White only needs to keep playing Na3, Nb1 and Black can adjust the parity.

Also "somewhat obvious" I would have thought.

This is less obvious, but I think it's illegal.

Black's f and g pawns have made two captures. The g pawn must have captured on h6, because otherwise three of White's pieces would need to be captured on light squares and one of them is a dark squared bishop.

That means both must have captured as well as the b7 pawn.

White cannot offer anything to capture until Black moves a piece to c3 or g3 or g4, so that piece was not the dark squared bishop.

So before White can offer any capture Black would need to have played c6.

But then the piece that Black offers must be the queen because no other missing pieces can escape the back rank. It must immolate itself on c3 because White's dark squared bishop is required at h6 and it would be hemmed in if the Black queen were captured at g3 or g4. That leaves one of White's rooks to be captured on a6 and one on g6. Also one of Black's rooks to be captured at b3 and one at g3 or g4.

Black's light squared bishop can reach b7 only after the White rook is captured on a6. Before that Black's queen's rook is restricted to a8 and b8. At the time of capture the Black knight on b8 must already be in situ otherwise it couldn't get there. That means the queen's rook must be on its original square and remain there or have been captured.

But both rooks are required to effect the captures by White's pawns on b3 and g3 or g4.

Nice one.

Black pawns can't have captured because White rooks never got out.

By the time both pawns are on squares shown both knights are on final squares otherwise no route to those squares.

Pawn on g3 must have moved from g4. If that was last pawn to move to White's third rank White would have no moves and BRg4 couldn't get there. Only White pieces that could move after g3 pawn in place would be WR on a1 or b1 and WN that finishes on a1. Only possible last move for White is then Na1 from a3 but BK couldn't have been on a5 at that point because it would be in check. Black can't make both BK a5 and b3 after White's last move.

I agree. Here's my try at an illegality proof.

All the knights are still on the board, so no other pieces can leave their back ranks until one side has made a pawn move that is not a capture. The only pawn with a useful move like that is at g7. The white bc pawns have to cross-capture to let the a1 rook out to be captured. Ph2-h3 doesn't release anything. Pc7-c6 takes away the square the b8 knight needs to return home after the b7 pawn captures to release the a8 rook. Pf7-f6 keeps that pawn from ever reaching g6 or h5.

After Pg7-g6 releases the f8 bishop, all three of Black's available captures will be pawn captures on light squares, so the c1 bishop will never be captured. But it is no longer on the board, so the position is illegal.

Rather more elegantly put.

You don't mention why the a8 rook could not be captured at home though when you rule out c6

(with the BN on b8 remaining unmoved).

I tried it

I did not succeed

I think not

I thought it was obvious, but it wouldn't have hurt to mention that of White's four captures (three by pawns, one by the queen at c7), none was of a black piece at home.

I guess it seemed obvious to me from the proof game I had made for the original (legal) position with the h7 pawn at h6.

MARattigan explained at #8413 why it's "somewhat obvious" that the position is legal. For fun, you might want to try making a proof game for my position.

Just nitpicking.

I can't complain about that, since I do it myself.

I cannot I have skill issue

Then I'll make one and you can see if you understand how it works.