When playing Chess4 with two or more players, the chess pieces are placed on the game board with all the Kings at the left of their Queens. The chess pieces must be placed on the first two rows of the outer edge of the game board.

The coordinates below are based on the following assuptions:

1. The board is laid out as a “plus sign”, with the “bottom of the plus” being toward/next to the viewer
2. As with traditional chess, White's pieces are placed on the two ranks closest to the viewer (M and N)
3. As with traditional chess, Black's pieces are placed on the two ranks furthest from the viewer (A and B)
4. There is considered to be a 3×3 cell area in each corner which is “off the board”. In other words, Black's King-side Rook is in square A4 because A1-A3 are “off the board”.

The colors always play in specific order: White plays first, then clockwise to Silver, followed by Black, and finally Gold.

Players can only move their Chess pieces on their turn. If a player is placed in check, when three or more players are playing, that player must wait until their designated turn before that player can respond to the threat. In the event a player is placed in check, and a player following moves into a position causing the player in check to be checkmated, the King is awarded to the last player (who actually checkmated the King). If Method 2 of play is being used, a player who has captured another player's King can only play those pieces on the turn of the player who has been checkmated and must continue to move those pieces on that player's turn only.

Pawns move forward only, unless attacking in a diagonal-forward manner. Pawns cannot attack diagonal in a backward manner. In the event a Pawn reaches King's row to the left, right, or directly across, that Pawn shall receive all the privileges of a Pawn reaching King's row during a traditional Chess game.

The traditional rules of Chess apply when playing Chess4. However, there are two methods in which Chess4 can be played:

1. Method 1 - Once a King has been checkmated, that player is out of the game, the remainder of the Chess pieces are left on the game board and play continues.
2. Method 2 - Once a King has been checkmated, the player who checkmated the King is now able to use the remaining Chess pieces of the captured King against the remaining opponents.

Players can choose partners for team play. Each partner can sit across from the other. For a more challenging game, players should not inform their partners as to what pieces to move or alert their partners when pieces are in danger. Two players can also play with two sets of Chess pieces each for a more challenging game.

1. A player is only eliminated by checkmate if he is checkmated at the beginning of the checkmating player's next turn.
2. An eliminated player's pieces are not considered “active” when assessing check/checkmate/stalemate.
3. An eliminated player's pieces cannot be “jumped over”, but they can be captured (and thus removed from the board), including the King.
4. It is an illegal move to capture an active player's King.
5. A player may not move his King into check (even if the checking piece will be forced to move before the player's next turn).
6. For purposes of determining which player gets “credit” for eliminating another player under Method 2, the following rules apply:
   1. Checkmate occurs at the beginning of the eliminated player's turn.
   2. If multiple players checkmate the King (two players' Knights, for example) then credit goes to the first player whose move resulted in a checkmate.
   3. If at any time the checkmated King becomes un-checkmated, the situation resets and credit goes to the first player who re-establishes the checkmate. For instance, if Gold moves his Queen to checkmate Black, and White captures Gold's Queen with his own Queen, then White has removed the checkmate (however momentarily) and reapplied it…so White would get credit for eliminating Black. However, if after White's move Silver moved his Queen to the same place (capturing White's Queen) then that would reset the checkmate a second time and ultimately give Silver credit for the elimination.
7. En passant
   1. En passant is also valid for perpendicular pawns. In other words, given a standard starting position, consider the following: “1. L11 … … K11”. This is a slightly different situation from standard chess, since it's still potentially possible for White's pawn to capture when/if Gold's pawn moves to K10. However, I'd say the en passant rule should still apply and White should be able to capture with “2. K12” (but not “2. K10”; you must always capture “behind” the captured pawn).
   2. If a player chooses to play en passant (regardless of whether the opponent's pawn is parallel or perpendicular), he must do so on his first opportunity. This is similar to the standard rule, with the following Chess4 clarification and extension:
      1. Other opponents' moves may occur between the move of the passing pawn and en passant, as long as the en passant is performed at the earliest opportunity. For example, if Silver passes a Pawn on White, White does not lose the opportunity to en passant simply because Black and Gold play their moves before White's next move.
      2. In the case of double en passant (for example, both Silver and Gold pass a pawn on White in the same round of turns), White may play en passant on both as long as he plays both moves on his next two turns. White chooses which passing pawn to capture first. The other passing pawn may not capture on this subsequent turn, but it may move; if it does, White may still capture (White's pawn's destination square remains unchanged).
      3. In the case of triple en passant, equivalent rules apply. White may capture all three passing pawns as long as he plays all three captures on his next consecutive three turns. The second and third passing pawns may move (but not capture) on their subsequent turns, but can still be captured (again, White's pawns' destination squares remain unchanged).

1. A player may castle a King acquired from an eliminated player if the eliminated player's King and castling Rook have not moved away from their starting positions since that player was eliminated. In other words, if Silver eliminates Black after Black has moved his King and/or his Rooks, Silver may still castle if he moves both the King and the castling Rook back to their original places. Neither piece may move “away” (either by rank or file) from its starting position (each move of that piece must bring it at least one rank or file closer to, and no ranks or files further from, its starting position). Once either piece is on its starting location, it may not move again (except to perform the castle) or castling is again illegal.
2. A player only retains control of one set of opponent's pieces. If a player eliminates a second player (and another player remains in the game) then that player must immediately make a permanent decision as to which eliminated opponent's pieces he will control. The other opponent's pieces remain on the board, but are static as with Method 1. The player may not choose to give up his own pieces to control both eliminated opponents'.
3. A player may only choose to control the piece of an opponent he directly eliminated. In other words, if Gold eliminates Silver, then Black eliminates Gold, Black may not choose to control Silver's pieces.
4. In cases of “discovered check” or “discovered checkmate”, credit for the move goes to the player who made the move that created the situation, even if the attacking piece belongs to an opponent. For instance, if Black creates a discovered checkmate by moving his Knight, which creates checkmate on Silver's King by revealing a discovered attack by Gold's Queen on Silver's King, then Black would get credit for eliminating Silver (even though Gold's Queen was actually performing the attack).
5. An eliminated player's King no longer has special significance for winning the game. If an opponent controls an eliminated player's King, he is not obligated to move it out of check (in fact, an eliminated player's King is never considered to be in check), nor will he incur any penalty if it's captured (other than the obvious loss of the piece).

As we play through our first full Chess4 game, I'm trying to work up a Chess4-specific PGN sub-format. It won't be directly usable by current PGN parsers of course, but any human reading the file (who's familiar with Chess4 and PGN files) should immediately understand what's going on. My goals with the format are to maintain that familiarity while requiring the smallest adjustment possible if a piece of software ever did want to support the format. The example below just goes through the fourth turn, so I'm not constantly filling this wiki page's edit history up by adding moves; once the game finishes I'll update the whole thing at once and until then you can see the current PGN for the game in progress.

firstgame.pgn

[ White                "Blinky"                                               ]
[ Silver               "Pinky"                                                ]
[ Black                "Inky"                                                 ]
[ Gold                 "Clyde"                                                ]
[ Date                 "2013.09.18"                                           ]
[ Date-Started         "2013.09.18"                                           ]
[ Date-Ended           ""                                                     ]
[ White-Eliminated-By  ""                                                     ]
[ Silver-Eliminated-By ""                                                     ]
[ Black-Eliminated-By  ""                                                     ]
[ Gold-Eliminated-By   ""                                                     ]
[ Result               "1/4-1/4-1/4-1/4"                                      ]
 
1. k7      { M7  > K7  } { White  }
   Nk3     { J1  > K3  } { Silver }
   d8      { B8  > D8  } { Black  }
   h11     { H13 > H11 } { Gold   }
 
2. Bk9     { N6  > K9  }
   Nd3     { E1  > D3  }
   Nc6     { A5  > C6  }
   Nf12    { E14 > F12 }
 
3. k10     { M10 > K10 }
   e4      { E2  > E4  }
   c4      { B4  > C4  }
   Qxk10   { G14 > K10 }
 
4. l9      { M9  > L9  }
   Nf4     { D3  > F4  }
   d4      { C4  > D4  }
   Qxk9    { K10 > K9  }
 
{ -----------------------------------------------------------------------------
 
Deviations from standard PGN:
 
  - The "Result" header is in a different format (specifics TBD)
  - The '#' and '##' notations for check and checkmate are now followed by
    one or more of the letters "wsbg" to denote which color(s) has been
    checked or checkmated
  - The 'o-o' and 'o-o-o' castling notations have been extended to include
    the color of pieces used (in case a player is castling another eliminated
    player's pieces)
  - A new notation to specify an untaken turn (by an eliminated player)
 
 
Example move notations (Chess4 extensions marked with '[*]'):
 
  k7           - Pawn moves are specified with just the destination square
  gxk7         - Pawn capture with disambiguating origination file
  8xk7         - Pawn capture with disambiguating origination rank
  g8xk7        - Pawn capture with disambiguating origination rank and file
  Nl13         - Knight move
  o-o-w        - King-side castle using White's pieces [*]
  o-o-o-s      - Queen-side castle using Silver's pieces [*]
  Nji12#g      - Knight (on file J) moves to I12, checking Gold [*]
  Qf3xb7#g##bw - Queen on F3 captures on B7, checking Gold and checkmating
                 Black and White [*]
  Qxb9###      - Queen captures on b9 and checkmates all remaining players [*]
  0            - Placeholder for a turn not taken due to player elimination [*]
 
----------------------------------------------------------------------------- }
 
{ EOF }