The Origin of Chess960

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Fischer random chess employs the same board and pieces as classical chess, but the starting position of the pieces on the players' home ranks is randomized, following certain rules. The random setup makes gaining an advantage through the memorization of openings impracticable; players instead must rely more on their skill and creativity over the board.

Randomizing the main pieces had long been known as shuffle chess, but Fischer random chess introduces new rules for the initial random setup, "preserving the dynamic nature of the game by retaining bishops of opposite colours for each player and the right to castle for both sides". The result is 960 unique possible starting positions.
In 2008, FIDE added Chess960 to an appendix of the Laws of Chess. The first world championship officially sanctioned by FIDE, the FIDE World Fischer Random Chess Championship 2019, brought additional prominence to the variant.

Setup

Before the game, a starting position is randomly determined and set up, subject to certain requirements. White's pieces (not pawns) are placed randomly on the first rank, following two rules:

1.The bishops must be placed on opposite-color squares.
2.The king must be placed on a square between the rooks.

Black's pieces are placed equal-and-opposite to White's pieces. (For example, if the white king is randomly determined to start on f1, then the black king is placed on f8.) Pawns are placed on the players' second ranks as in classical chess.

After setup, the game is played the same as classical chess in all respects, with the exception of castling from the different possible starting positions for king and rooks.

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Creating starting positions

There are 4 × 4 × 6 × 10 × 1 = 4 × 4 × 15 × 4 × 1 = 960 legal starting positions:

• 4 light squares for one bishop;
• 4 dark squares for the other bishop;
• 6 remaining squares for the queen and 5! / (3! × 2!) = 5 × 4 / 2 = 10 ways to place the two (identical) knights on the remaining 5 squares,
  • or, equivalently, 6! / (4! × 2!) = 5 × 6 / 2 = 15 ways to place the two (identical) knights on the remaining 6 squares and 4 remaining squares for the queen;
• 1 way to place the two rooks and king on the remaining 3 squares, since the king must be between the rooks.

Usually, the players accept the conditions of the organizer to generate the starting position with software, as it was used in the 2019 World Fischer Random Championship.

If the software is not available or the players don't accept it, there are many ways to generate a number in the appropriate range with equal probability; this number is then used as an index to the Fischer Random Chess numbering scheme.

In 1998, Ingo Althöfer proposed a method that requires only a single standard die.
(Re-roll if needed to get values in the range 1–4 or 1–5).

960 choices can be obtained in several ways by combining polyhedral dice without re-rolling; for example 4×12×20 or 6×8×20 or 8×10×12.

Toss a fair coin four times; record each ‘head’ as 0 and each ‘tail’ as 1. If the first four bits are all 1, start over; otherwise, continue with six more tosses. Now you have ten bits ranging from 0000000000 to 1110111111 (decimal 959) inclusive. The last four bits determine the positions (0-15) for the bishops, the first four bits determine the positions (0-14) for the knights and the two bits in the middle determine the position (0-3) for the queen.

Shuffling marked objects (cards, pieces, pawns, dominoes tiles, scrabble letters) and use the permutations. For example, shuffle 14 marked cards a,b,c,d,e,f,g,h, N,N,Q,R,R,R and place them, in this random order, separated into three rows:

• For the black squares (a,c,e,g);
• For the white squares (b,d,f,h);

The first card of each row determines one bishop's place.

The remaining cards Q,R,R,R,N,N place the queen, rooks and knights respectively in the remaining squares; the king must be between the rooks, so it takes the middle of the three 'R' squares.

Castling rules

As in classical chess, each player may castle once per game, moving both the king and a rook in a single move; however, the castling rules were reinterpreted in Fischer random chess to support the different possible initial positions of king and rook. After castling, the final positions of king and rook are exactly the same as in classical chess, namely:

• After a-side castling (queenside/long castling in classical chess), the king finishes on the c-file and the a-side rook finishes on the d-file. The move is notated 0-0-0 as in classical chess.
• After h-side castling (kingside/short castling in classical chess), the king finishes on the g-file and the h-side rook finishes on the f-file. The move is notated 0-0 as in classical chess.

Castling prerequisites are the same as in classical chess, namely:

• The king and the castling rook must not have previously moved.
• No square from the king's initial square to its final square may be under attack by an enemy piece.
• All the squares between the king's initial and final squares (including the final square), and all the squares between the castling rook's initial and final squares (including the final square), must be vacant except for the king and castling rook.

FIDE's recommended procedure for castling unambiguously is first to move the king outside the playing area next to its final square, then to move the rook to its final square, then to move the king to its final square. Another recommendation is to verbally announce the intent to castle before doing so.