Illegal Position Contest!

What's a retro-problem?

A problem whose solution requires analyzing the possible moves that could have led to the problem position.

Nobody ever specifies a 50-move rule move count but there are quite a lot of compositions (e.g. by N.Plaksin) where the diagram position is provably near the 50-move limit which therefore affects the solution. Btw, the 50-move convention is not completely clear and appears to accept that a diagram position is legally past that count but at that point immediately draws. Which is weird since "how did you get there?"

Everything on this site is named "puzzle" and nobody knows what that means in terms of the rules and conventions. What we know for sure is that "the game rules are not enough" to solve just about any puzzle. For instance, nobody can prove that a diagram is not the 3rd occurrence of the same and therefore a (claimable) draw. So you need "extras" and the field of composition provides them. More important: nobody else provides them so the composition conventions are about the only place to go. And it does not only affect the 50-M rule but also repetitions, castling, e.p. moves and "on-move" decisions. Plus some other things following from the interaction of some conventions with with the "dead positions" rule.

The priority order of applicable rule sets is (first 3 for compositions):

1. The composition author can change any Codex- or FIDE-rule
2. The Codex overrides some FIDE-game-system-rules - like removing the 50M restriction for common problems
3. The Codex appends choice rules to fill in the "unknowns" - like castling right.
4. Chess.com rules override some FIDE-game-system-rules but have no defined impact on compositions.
5. FIDE-rules define how to play standard chess games and provide a basis for compositions.

I took some liberties in calling the #7721 position illegal, since I wanted it to belong in this thread. That the Codex says the 50-move rule applies is not the same as making the draw automatic. MARattigan's first reply that it's not really illegal is probably right. It might be better to ask why the position is a draw rather than why it's illegal, since by the time it's reached, either side could claim a draw.

Someone still needs to show that the 50-move limit has to be exceeded.

This is illegal ❌

Ah, I said it was unclear which means that nobody knows exactly what the Codex says or means. So you can take liberties if you like!

Because the 50-M convention has different parts and is confused, I'll give you my rational interpretation - which is the same as for the repetition convention which is much clearer.

1. The Codex provides a FIDE rule change with regard to the proof games for retro-problems which says (and has said for many, many years) that any proof game automatically draws at the 50M mark. No 75M, No claims, No arbitration, No choice. Game terminated. Just as for the repetition convention.
2. In a forward playing solution you can and must invoke the automatic 50M draw when no proof game exists where less than 50 pawnless and captureless moves were played in the stretch towards the "current position". Of course, the "current position" starts with the reference diagram but changes with every move of the solution played.

Note that the first prescription falls under category 2 (system rules) of the applicable rule sets and the second prescription under category 3 (choice rules).

There is much more to say about this in relation to the RA-type (pure retro-analysis) and whether or not the proof games in this topic come under the heading of the RA-type but I'm afraid that would only increase confusion here.

updated 7719

Oh you mean like this one:

White to move mate in 2.

I don't know what #7719 was before, but it's illegal now. White's first retraction to uncheck the black king had to be -Bc7xb8. It had to be an uncapture, or else there had to be a black unit at c8 to block queen checks, to give Black a previous move. The capture on b8 or c8, added to the 13 captures by the white pawns, means White needs 14 captures, and Black is only missing 13 units.

Too many black pawns (including extra pieces)

Too many White pawns

White pawn is on 8th rank but not promoted

No white king

2 black kings

White pawn is on 1st rank

White bishops are on the same color, while having all 8 pawns

White rooks cannot be on any rank except 1st because White's pawns are unmoved

White knight cannot be on that square (edit: it can)

All I can find :ο

But it can

updated 7719 again because of pa7-a6

yes

Now it's legal. But it may have changed again by the time I post this.

changed again

I'll wait to look at this one until EvinSung agrees not to change it again and post a new position if it needs to change.

#7719 is legal if the black king just captured a knight on a7.

i do not think so