The Chess Variant Pages

This page is written by the game's inventor, Jose Carrillo.


Chess8400 is not a new game.

Chess8400 is the subset of 8,400 starting positions in Pseudo-Modern Random Chess where the Bishops start in opposite color squares.

The remaining 6,720 (of 15,120 total) Pseudo-Modern Random Chess positions have the Bishops starting on same color squares.

Chess8400 is really Fischer Random Chess (Chess960) played in a 9x9 board, with the The Prime Minister as the 9th piece, and an extra pawn.

Chess8400 is similar to Prime Ministers Random Chess played on a 9x8 board, but the board for the latter was inspired by Capablanca's Chess.



All pieces are randomly placed in the player's first rank, with two restrictions:

-Bishops start in opposite color squares.
-The King must be between the two rooks.

The opponents' pieces are placed with direct symmetry across the board (White's piece at a1 is equivalent to Black's a9, White's b1 to Black's b9, White's c1 to Black's c9, and so on)

Pawns (9 of them) are placed on each player's 2nd rank.

There are 8,400 different legal starting positions in Chess8400.


Orthodox Chess pieces are used, plus a Prime Minister (Bishop+Knight) and a 9th Pawn per side.

Approximate Value of the Pieces on a 9x9 board

  • Queen - 11 points
  • Minister - 10 points
  • Rook - 7 points
  • Bishop - 4 points
  • Knight - 4 points
  • Pawn - 1 points

Practical Value of the Pieces (adjusted to an 8x8 board)

  • Queen - 9 points
  • Minister - 8 points
  • Rook - 5 points
  • Bishop - 3 points
  • Knight - 3 points
  • Pawn - 1 points

Kings and their Prime Ministers from a 1968 Gabriel Vicente Maura's Modern Chess set:

Homemade Prime Ministers

Knight crowned by small Bishop, Bishop crowned with small Knight, and a crownless King.

Knights crowned with the heads of Bishops.

The Prime Minister is the only piece that is able to give a checkmate on it's own. Sample Prime Minister's checkmates:


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Most Orthodox Chess rules apply, including the objective of the game, the way pieces move and capture, en passant, check, checkmate, and the various draw situations (i.e. stalemate, insufficient mating material, threefold repetition, 50-move rule). Pawns may promote to a Queen, Minister, Rook, Bishop or Knight. Modified Castling rules are below.


The Chess8400 castling is long to either side of the board and the rules are based on the Modern Random Chess rules.

In Chess8400, depending on the pre-castling position on the castling King and Rook, the castling manoeuvre is performed by one of these four methods:

  • Double-move castling: By on one turn making a move with the king and a move with the rook.
  • Transposition castling: By transposing the position of the king and the rook.
  • King-move-only castling: By making only a move with the king.
  • Rook-move-only castling: By making only a move with the rook.

After castling, the rook and king's final positions are exactly the same positions as they would be in Gabriel Vicente Maura's 1968 Modern Chess. Thus, after c-castling (notated as O-O-Oc), the King is on the c-square (c1 for White and c9 for Black) and the Rook is on the d-square (d1 for White and d9 for Black). After g-castling (notated as O-O-Og), the King is on the g-square (g1 for White and g9 for Black) and the Rook is on the f-square (f1 for White and f9 for Black).

This table shows where the King and Rook end up and the notation for each type of castling.

White castles a-sidec-castlingO-O-OcKc1, Rd1
White castles i-sideg-castlingO-O-OgKg1, Rf1
Black castles a-sidec-castlingO-O-OcKc9, Rd9
Black castles i-sideg-castlingO-O-OgKg9, Rf9

However, castling may only occur under the following conditions, which are extensions of the standard rules for castling:

  • Unmoved: The King and the castling Rook must not have moved before in the game, including a previous castling or a Bishop Adjustment.
  • Un-attacked: All of the squares between the king's initial and final squares (including the initial and final squares) must not be under attack by any opposing piece.
  • Vacant: 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.
These rules have the following consequences:

  • Castling cannot capture any pieces.
  • The king and castling rook cannot "jump" over any pieces other than each other.
  • A player may castle at most once in a game.
  • If a player moves his king or both of his initial rooks without castling, he may not castle during the rest of the game.
  • In some starting positions, some squares can stay filled during castling that would have to be vacant in Modern Chess. For example, after c-castling (O-O-Oc), it's possible for to have a, b, and/or e still filled, and after g-castling (O-O-Og), it's possible to have e, h and/or i filled.
  • In some starting positions, the king or rook (but not both) do not move during castling.
  • In some starting positions, castling can take place as early as the first move.
  • The King may not be in check before or after castling.
  • The King cannot move through check.
  • The King cannot jump over his own rook if and when said rook stands on a 'checked' square. This point is best ilustrated by the sample Fischer Random Chess (Chess960) position below where it's illegal for White to g-castle (O-O) with it's e-Rook, as the King would need to jump over an attacked square. Even though the e-square is occupied by a Rook, it is under attack (or 'checked') by Black's Rook on e8.

  • Castling in Chess8400 is symmetric to either side of the board. Chess8400 Castling is like the Orthodox long castling (O-O-O) but to either side.


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This 'user submitted' page is a collaboration between the posting user and the Chess Variant Pages. Registered contributors to the Chess Variant Pages have the ability to post their own works, subject to review and editing by the Chess Variant Pages Editorial Staff.

By Jose Carrillo.
Web page created: 2009-07-16. Web page last updated: 2009-07-16