Fischer Random Chess Rules

For those of you who want to "mix it up" a little. This does take everyone out of "book" and sends you into the unknown. Give it a try!

Starting position requirements

White pawns are placed as in standard chess. All remaining white pieces are placed randomly on the first rank, with the following restrictions:

the bishops must be placed on opposite-color squares
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. (Note that the king never starts on the a- or h-files, since this would leave no space for a rook.)

The starting position can be generated before the game by computer program, or chosen by the players by a variety of methods using dice, coin, cards, etc.

Determining a starting position

There are many procedures for selecting a starting position. A common one is that proposed by Ingo Althoefer in 1998, which requires only one six-sided die:

Roll the die, and place a white bishop on the black square indicated by the die, counting from the left. Thus, 1 indicates the first black square from the left (a1), 2 indicates the second black square from the left (c1), 3 indicates the third (e1), and 4 indicates the fourth (g1). Since there are no fifth or sixth positions, re-roll a 5 or 6 until another number shows.

Roll the die, and place a white bishop on the white square indicated (1 indicates b1, 2 indicates d1, and so on). Re-roll a 5 or 6.

Roll the die, and place the queen on the first empty position indicated (always skipping filled positions). Thus, 1 places the queen on the first (leftmost) empty position, while 6 places the queen on the sixth (rightmost) empty position.

Roll the die, and place a knight on the empty position indicated. Re-roll a 6.

Roll the die, and place a knight on the empty position indicated. Re-roll a 5 or 6.

This leaves three empty squares. Place the king on the middle empty square, and the rooks on the remaining two squares. Place all white and black pawns on their usual squares, and place Black's pieces to exactly mirror White's (so, Black should have on a8 the same type of piece White has on a1, except that bishops would be on opposite-color squares).

This particular procedure generates any of the 960 possible initial positions with equal chance. It uses an average of 6.7 die rolls. Note that one of these initial positions (rolled by 2-3-3-2-3 or 2-3-3-4-2) is the standard chess position, at which point a standard chess game ensues.
Rules for castling

Like standard chess, Chess960 allows each player to castle once per game, moving both the king and a rook in a single move. However, the castling rules were reinterpreted in Chess960 to support the different possible initial positions of the king and rook.

After castling, the king and rook's final positions are exactly the same as they would be in standard chess. Thus, after a-side castling (also called sometimes c-castling), the king is on the c-file (c1 for White and c8 for Black) and the a-side rook is on the d-file (d1 for White and d8 for Black).

This move is notated as 0-0-0 and is known as queenside castling in orthodox chess. After h-side castling (also called sometimes g-castling), the king is on the g-file and the h-side rook is on the f-file. This move is notated as 0-0 and is known as kingside castling in orthodox chess. It is recommended that a player state "I am about to castle" before castling, to avoid potential misinterpretation.

However, castling may only occur under the conditions listed below. The first two are identical to the standard chess castling rules. The third is an extension of the standard chess rule, which requires only that the squares between the king and castling rook are vacant.

Unmoved: The king and the castling rook must not have moved before in the game, including having castled.

Unattacked: No square between the king's initial and final squares (including the initial and final squares) may be under attack by an enemy piece.

Unimpeded: All the squares between the king's initial and final squares (including the final square), and all of the squares between the rook's initial and final squares (including the final square), must be vacant except for the king and castling rook. (An equivalent way of stating this is: the smallest back rank interval containing the king, the castling rook, and their destination squares, contains no pieces other than the king and castling rook.)

Observations
If the initial position happens to be the standard chess initial position, the Chess960 castling rules have exactly the same result as the standard chess castling rules. In some starting positions, some squares can remain occupied during castling that would have to be vacant in standard chess. For example, after a-side castling (0-0-0), it is possible that a, b, and/or e are still filled; and after h-side castling (0-0), it is possible that e and/or h are filled. In some starting positions, the king or rook (but not both) do not move during castling.