Illegal Position Contest!

The positions posted here are supposed to be illegal. You might want to post this legal position in the Shortest Proof Game thread.

Yeah, I was wondering how that was illegal.

CatRobots8, on what basis would your position be illegal?

(One of the many ways to show CatRobots8 the proof)

I'm not certain that this position is illegal. n9531l1 might have an answer on this.

EDITED: Just realised it's illegal. Let me move a pawn and remove a White rook instead. This should be borderline legal or borderline illegal.

I would say it's clearly illegal. The white b-pawn needs four captures to reach f8, the white h-pawn needs a capture, and White has to capture the c8 bishop at home, for a total of six captures, but Black is only missing two men. Not really borderline.

You could remove the f6 pawn, the b5 rook, the e5 pawn, and the g6 knight to make it borderline legal, or retain one of them to make it borderline illegal.

You made a good point there. I was focusing so much on the activity around the f8 square and trying to create the two bishops from promoted pawns that I failed to realise that White already had seven pawns to begin with.

I removed the four pieces which you suggested (and added the c8 bishop for clarity), and after thinking about it, it's actually still illegal due to these reasons

>> White still needs five captures total (four for the b2 pawn, one for the h3 pawn)

>> There are five missing pieces on the board at the moment, but subject to the condition below

>> The bishops on d5 and g1 must have been promoted (this was my consideration when I placed three pawns on the kingside initially)

>> The original f8 bishop likely stays there (though it could be something else which gets captured there)

>> So in effect, Black has only a rook, a knight and two pawns to offer (likely the a7 and e7 pawns, since the f7 pawn is required for obtaining the light-squared Black bishop -- via two captures of White pieces -- and the g7 pawn is required for obtaining the dark-squared Black bishop -- via two captures of White pieces).

When I return home at night (it's the afternoon now here), if I can somehow still remember by then, I will study the effects of removing the d3 White pawn or e4 White pawn and see how to make the resulting position borderline illegal - since the captures via the b2 pawn are too trivial now.

Now that I am quite inactive on this thread (as does @BigDoggProblem and other older regular commenters) and I could not find novel ideas for new illegal positions, I guess I have now become a thing of the past on this thread.

Your new position is legal. Without adding the c8 bishop, it's also legal.

Why illegal? The white pawns made three captures on light squares, but the three missing black men include a dark-squared bishop.

the black pieces are on the white side of the board and visa versa

I did that position first

Good observation which I missed! Since the position is already borderline legal, then removing the d3 or e4 pawns would make the position very much legal, so I will abandon my attempt to modify the position.

Why illegal? White has no legal retraction to uncheck the black king. Uncastling gives a position where the black king has no path to reach h1.

Why illegal? White has no legal retraction to uncheck the black king. Uncastling gives a position with a rook at a2 that had no way to get there. Do you see why it's still illegal if the a2 rook is removed?

#6431 is illegal, double check, also checkmate and illegal... What would be white's last move if it was legal ?