The caption of the last diagram is "Legal but non-reachable position with 271 moves for White. Corner queens can be replaced with bishops." There are 24 white queens.

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contravariant 7 hours ago | parent | next [-]

That's the difference between illegal an unreachable. To reach that position you'd need to start from a different point (start both sides with 16 pawns or w/e), but you wouldn't need to break any other chess rules.

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systoll 5 hours ago | parent [-]

If you can start from any arbitrary different point, you can just start from the point you’re intending to reach, and don't need to break any other rules.

As per FIDE rule 3.10.3 "A position is illegal when it cannot have been reached by any series of legal moves". The position isn't legal per FIDE rules.

Beyond there being too many queens… black could not possibly have made the last move. For white to have any moves right now, the last move must have been black moving the king to H8. But G8, G7, H7 are all occupied, so where could the King have moved from?

	▲	zuminator 3 hours ago \| parent [-]
		Legality is a long standing term of art used by chess problem creators. Essentially it means a position withtwo kings on the board, non touching, and not both in check. And no pawns on first or eighth ranks. It has nothing to do with whether the position is reachable from standard chess rules. Along came FIDE in 1999 with its standardized nomenclature but that doesn't invalidate the terminology used by chess problem creators in their own work.

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Bootvis 7 hours ago | parent | prev [-]

I understand this as follows:

Legal: doesn’t break the rules of chess. For example: no pawns on the eighth rank or in check when it’s not their turn.

Reachable: there is a series of moves that lead to this position from the standard starting position.