Chess960

What this variant is

Chess960 keeps every rule of standard chess and changes only where the pieces start. The pawns stay on the second and seventh ranks. Behind them, the eight pieces are arranged in one of 960 legal ways, chosen at random before the first move. This site follows the international federation’s appendix on the variant, the same one used in over-the-board competition.

When a game is created, the server picks one of the 960 arrays by its Scharnagl number, using a cryptographically secure random generator, so nobody can predict or steer the draw. Both players receive the same array. Black’s pieces mirror White’s down the files exactly as in standard chess, so the position is symmetrical and the game starts even.

How the array is built

Not every shuffle is allowed. Two conditions cut the 40,320 possible orderings of eight pieces down to 960.

• The two bishops must stand on squares of opposite colours, so that between them they cover the whole board, exactly as in standard chess.
• The king must stand somewhere between the two rooks, so that castling in both directions still makes sense.

The standard array is one of the 960, so an ordinary game of chess can and occasionally does come up. Everything else about a piece is unchanged: knights leap, bishops keep their colour, rooks and queens slide, and pawns behave exactly as they always have, including the two-square first move, capture in passing, and promotion to any of the four pieces.

Castling

Castling ends in the same place it always does. On the kingside the king finishes on g1 or g8 and the rook on f1 or f8. On the queenside the king finishes on c1 or c8 and the rook on d1 or d8. That is true no matter where the two of them began, so one or both may barely move, and occasionally the king appears not to move at all.

The conditions are the familiar ones.

• Neither that king nor that rook has moved earlier in the game.
• Every square between the king’s start and finish, and between the rook’s start and finish, is empty except for the two men doing the castling.
• The king is not in check when you castle.
• The king does not pass over, or land on, a square an enemy man attacks.

Because the king sometimes ends up on a square it might have reached with an ordinary move, castling has to be entered unambiguously. Play it as a king move to the destination square, g or c, or as a king move onto its own rook. Either input is accepted.

Winning and drawing

Exactly as in standard chess. Checkmate, resignation and timeout win; stalemate, agreement, threefold repetition, the fifty-move rule, a dead position, and a timeout against an opponent with no mating material all draw. Repetition and the fifty-move rule are applied automatically for both sides.

Sources

Everything on this page is written from scratch. These are the works it leans on, and what each one settles.

• FIDE Laws of Chess, Appendix II (Chess960)

  The competition rules for the variant. It fixes the 960 legal arrays, the castling destinations, and the requirement that castling be entered in a way that cannot be confused with a normal king move.