This variant features a less traditional composition of the royal families.
The setup is the same as that of orthodox Chess, except that the white Queen is replaced by a second King, and the black King by a second Queen.
All pieces move the same as in orthodox Chess. The royal Queens are not allowed to move into or through check, though.
Unless explicitly contradicted, all FIDE rules apply.
The white Kings are 'extinction royalty', i.e. white can expose his Kings to capture as long as he has two, and the normal checking rule applies only when he has a single King left.
In contrast, the black Queens are 'absolute royalty': moves that expose either of them to capture are not legal. It is also illegal for a Queen to 'pass through check', i.e. continue a sliding move after having reached a square that was (pseuo-legally) attacked by an opponent. There is one exception to this: a Queen can capture the last-remaining King from a distance.
Pawns can promote only to Rook, Bishop or Knight.
Both white Kings can castle with the normal conditions of virginity, free path and not passing through check. Castling moves them 2 steps towards the Rook they castle with. (I.e. the King at d1 ends on b1 or f1.) Black cannot castle.
Even though they cannot move through check the Queens are more powerful than the Kings, as (apart from the possibility to castle) their move is upward compatible with that of a King. This advantage is offset by the rule that white can afford to sacrifice one King, while black must always hang on to both Queens. It is not guaranteed that this perfectly balances the game, but it should be close.
The current version of the diagram 'greys out' all moves of a royal piece to squares where it could be captured. You should consider this just a reminder, and not as the final verdict that such a move is illegal. In particular it should not deter you from playing the greyed-out King moves when you still have a second King. The AI is fully aware of the rules, and won't claim a win on capturing a King when you still have a spare one. Perhaps future versions of the diagram will use precise legality testing for deciding when to grey out moves (So that it would also work for moves of pinned pieces.)
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By H. G. Muller.
Web page created: 2021-05-02. Web page last updated: 2021-06-05