The three classical exceptions, rebuilt for a board where pieces don’t have fixed identities.
Castling, en passant, and promotion all survive on the quantum board — but each had to be reinterpreted for a world where a piece is a set of possibilities rather than a known type. Each becomes more flexible and a little stranger. Understanding them turns three “special cases” into real tools.
Classical castling needs your king and a rook on their original squares, neither having moved. On the quantum board there is no such memory and no privileged square, so castling is redefined by possibility: it applies to any two pieces on your back rank whose lists still include both Rook and King. No move history required.
The two pieces move simultaneously toward each other along a clear path and finish on the two most central empty squares between them. Afterward, both are exactly Rook-or-King — the move measures them into that pair. Two things to plan around:
En passant also generalizes. When an enemy piece makes a two-square straight first-move advance while Pawn is still among its possibilities — and because no square is privileged, that advance can come from any rank — you may capture it en passant on your very next turn, exactly as in classical chess: your capturer lands on the square the enemy skipped over.
The measurement cuts both ways. The captured piece collapses to a Pawn (that’s what made it en-passant-able), and so does your capturing piece — only a pawn captures en passant, so performing the capture confirms your piece as a Pawn too. It’s a genuine trade of information: you remove an enemy possibility set, but you spend your own piece’s mystery to do it. The window is one move only, as always.
Promotion is the subtlest of the three, because a piece reaching the far rank might not have been a Pawn at all. So promotion isn’t an assertion — it’s a branch. A piece that still includes Pawn and reaches the far rank promotes immediately, but only in the worlds where it was a Pawn:
A promoted piece wears a solid bar beneath it for the rest of its life. Heals never return Pawn to it. And yes, nine queens are perfectly legal — eight of them are pawns in disguise. The strangest consequence: a promotion can snap back. If your other pieces are later confirmed to be all eight pawns, the census proves this piece was never a Pawn, the promotion branch dies, and the piece reverts to its surviving non-Pawn identities. Promotion is a bet the census can later settle either way.