Illegal Position Contest!

Illegal. The position below, where a white queen has just moved to g4, shows the only legal way to reach the final position without the added pawns. With the added pawns, every square on the sixth rank is guarded by an unmoved black pawn, and the white king can't pass the fifth rank.

I would like other people to post challenging illegal positions, but I can still keep posting if no one else wants to do so.

Has anyone come up with a position which is not reachable, but ONLY due to the 50 move rule?

I guess the main issue with creating or finding such a position is pawn moves reset the move counter, and such a position needs to leave absolutely no possibility that there has been a recent capture or pawn move, or that if there has, that there was a 50 move gap at some point during the game.

This makes bishops a slightly headache-inducing piece, as they have to remain in place (and hold other pieces in place as a result) unless a capture or pawn move was made. Fortunately, we do know that every move involving the bishop or the pieces it had blocked must have happened after that capture or pawn move. Unfortunately, any position that lets a rook fully escape is not possible to determine whether the pawn advanced one square or two.

I found this position that T. Volet created. Here’s the link: https://www.janko.at/Retros/Glossary/FiftyMoves.htm

Or here as a board

First part: game here ended by 50 move rule

Second part (as you can see the position is legal, but would have ended by the 50 move rule)

Interesting post.

Your final position would be illegal under the 75 move rule under FIDE competition rules, but not under USCF rules where the application of the rule depends on the mood of the TD.

The 50 move rule would make a position unreachable only under some software GUIs that work according their own rules (arguably not chess at all) and make the 50 move rule mandatory. Under FIDE competition rules or USCF rules the 50 move rule wouldn't render any positions illegal because it applies only if it's claimed and under FIDE basic rules since 2017 it also couldn't because it's not there. 

Your proof game doesn't prove that the position can't be reached within the 75 move rule, of course.

I’ve improved the proof game but after 32. hxg4 there are 80 non-capture, non-pawn moves that follow. I don’t think it is possible to do 75 moves

https://www.janko.at/Retros/Glossary/FiftyMoves.htm

The link describes why the rook(s) must follow the long path, but is not very descriptive about why b7-b6 unlocks the position, and more. Here’s my analysis:

All the pawns have captured the missing pieces. bPb6 must come from b7, otherwise there will always be a black pawn on a7 or b6, preventing a pathway for bBa7. So retracting c6-c5 is illegal because it forces bPb6 to come from a7. But retracting b7-b6 immediately is illegal for the bBc8 cannot get captured by the white pawns, which made 4 captures. So we must uncapture this bishop somewhere. (The link shows where)

The black king can only get there from the whole b3. From b3, the black king must come from a4, b4 or c4. But c4 requires wPc4 to retract, and it must be from b3. This makes the black king unable to get there. Now a4 and b4 will be shown in the next paragraph.

We must at some point retract b7-b6 to unlock the position so the wNa4 and wBa3 can escape, as well as the black king. Retracting a move with bPc5 requires bPb6 to move. If we retract c2-c3, then the black king must escape the first 3 ranks, otherwise he cannot escape. If the black king came from b4, then either the white bishop is occupying b4 or is on a3 or a5, so wPc3 and the wB give illegal double check. If the king came from a4, then the wNa4 must retract, and since that retracted before c2-c3, the wN must escape via b6. This is the case for the black king retracting from b4 and a4 even if white does not retract c2-c3.

So before the retraction of b7-b6, the wNa4 is frozen. This also means that bNa8 is frozen. We cannot retract d2xe3 before b7-b6 as the wB cannot get to c1, and of course it could not be promoted. Therefore wNd1 is also frozen in position. These frozen knights create a winding path for the rook if it were to travel, which it must because it acts like a shield for the wK against bRe7. More about that explained by the link

Your reasoning seems good to me, but MARattigan can refute it by showing a proof game that reaches the position within the 75-move rule.

It seems I have refuted my own reasoning. However, it cannot be done shorter than 75 moves after hxg4: the white bishop (b4) and king are optimally placed.

This seems illegal to me. To avoid a collision on the a1-h8 diagonal, the white b-pawn has to reach f7 (after four captures on dark squares) before the g7 pawn starts making its captures to reach b2. So the black f8 bishop and both black rooks will be trapped on the back rank, and Black only has three pieces for the white b-pawn to capture.

Actually both rooks can come out if bBf8 was captured at home, but there will be lack of captures for wPh5.

You're right, my proof was incomplete and should have noted that the captures by the white b-pawn and e-pawn account for all of White's captures, so no black piece can be captured at home.

I didn’t think about the rooks before. I thought bBf8 could not get captured by any of the wPs (as wPh5 captures on light squares), but the rooks also could not get captured unless we get rid of bBf8.

It could be a proof game problem if the white e-pawn only got to g4.

This position, made by me, requires a lot of analysis to prove its illegality.

Black has made three captures by the bfg pawns and one capture of the white a-rook on the first or second rank, for a total of four captures, but White is only missing three men.

Don't forget a common trick in retros - cross-captures.