In order to avoid lengthy statements of the rules of any particular chess variant, the assumption is usually made that "all the laws of orthodox chess apply unless otherwise stated". Accordingly we need a clear statement of what these laws of orthodox chess are. This page provides such a statement. However, a simple duplication of the latest version of the Laws of International Chess as laid down by the Federation Internationale Des Echecs (FIDE) would not be sufficient. The reason for this is that many situations can arise in chess variants that are simply not possible in the orthodox game, so a more widely applicable statement of the rules is needed.
The idea of this statement of the laws of variant chess, besides setting a standard, is also to provide a method of classifying variants according to the laws which they vary. I do this by coding the laws in six sections lettered A to F, each divided into five sections numbered 1 to 5. Then the type of a chess variant is specified by the letters and numbers of the laws that it alters.
A Players and the Turn to Play
A1. Number of Players. Orthodox chess and most variants are games for two players. There are popular variants for three or four players, and less well known games for more than four, or even for one or none (the latter are more usually considered to be patiences or puzzles).
A2. Control of Pieces. Each player is assigned a distinctive set of pieces whose moves are under his control. This is usually shown by the colour of the pieces, though in Shogi it is shown by the direction in which the shaped tiles point. In orthodox chess the two colours and the two players are conventionally referred to as White and Black, although the pieces are often not in fact of those colours, being for example yellow and brown. In some variants there may be pieces of other colours, for example neutral pieces, that can be moved by either player, or by neither player (their moves, if any, are consequences of moves of the players' pieces). The assignment of colours is by agreement between the players or by some chance method.
A3. First to Play. In a single game some chance method should be used to decide which player has the first move. In orthodox chess this decision is combined with the selection of colours; White always taking the first move (though historically this has not always been the case). In games with more players the correspondence of move-order with colours is not so rigidly adhered to. In a single game the assignment of first move is by some chance method (e.g. one player shuffles a white and a black pawn between his hands, behind his back, and the other player chooses one hand and accepts the colour indicated by the pawn thus selected, white indicating first move). In tournaments or series of games the turn to take first move usually rotates to ensure that each player has an equal chance.
A4. Opposition. The players are also termed "opponents" and are expected to "oppose" each other's actions, though how this can be enforced is debatable, as the phenomenon of the Grandmaster Draw testifies; where leading players meeting in a tournament agree a draw so as to save their energies to meet lesser opponents in the next round. Help-play is an important type of play in an extensive group of variant chess problem stipulations, such as Helpmate, Helpstalemate, Serieshelpmate. In these stipulations, one player helps the other to reach the specified finale, though the help does not extend to making illegal moves such as moving a king through check.
A5. Obligation to Move. At his turn to move a player must make a move. This applies, even if all available moves are to his disadvantage; this is the situation termed Zugzwang (German for 'move-compulson'). In some games Null Moves are possible, where no change occurs to the position, e.g. in cylinder chess where a rook can move round the cylinder and end where it started. Such null moves are usualy only possible under very special conditions. In some variants this rule needs to be extended to prohibit Reversals, that is moves which simply reverse the action of the opponent's last move (which is of course impossible in orthodox chess). This is necessary for instance in Dynamo chess.
B Board and Opening Position
B1. Type of board. In orthodox chess the board is an arrangement of square-shaped cells in ranks and files. However there are many variants in which the cells are hexagons in honeycomb a pattern or even cubes in three dimensions, and other boards with cells of various different shapes.
B2. Size and Shape of board. The orthodox board is square with 8 cells in each rank and file, total 64. Many popular variants use a wider board, say 8 by 10 or a larger board, say 10 by 10; it is also possible to use cross-shaped boards, or boards with holes. Similarly, honeycomb-type boards can be hexagonal, star-shaped or lozenge-shaped in outline. Chequering.
B3. Home area. In orthodox chess the board is said to be "placed between the players". This really means that one edge is assigned to White and the opposite edge to Black.
B4. Forces. In orthodox chess six types of piece, King, Queen, Rook, Bishop, Knight, Pawn are used, each represented by a distinctively shaped token; or alternatively by a counter bearing its symbol. For the purpose of starting the game we do not need to know what the different shapes mean, only how to distinguish between them. In variants other types of piece may be introduced. In orthodox chess both players have the same forces, consisting of K, Q, 2R, 2B, 2N, 8P. In variants the numbers may be different, and the players need not necessarily have the same forces.
B5. Opening Array. Each player places his pieces in his home area in a specified array. In orthodox chess the 8 pawns are on the second rank and the first rank is occupied by the other pieces in the sequence RNBQKBNR.
C Pieces and Moves
C1. Moves. A normal move is the transfer of a piece from one cell to another cell that is vacant. Such a move can be specified by coordinates (r,s) indicating a move to a cell r ranks and s files away. A piece that can make any move (±r, ±s) or (±s, ±r) are termed leapers.
C2. Captures. A capture-move is like a normal move but goes to a cell occupied by an opposing piece, which is immediately removed from the board.
C3. Rides. Certain pieces, called riders, can move to any cell at distances (±nr, ±ns) or (±ns, ±nr) provided the intermediate cells passed over are not occupied by any piece of either colour.
C4. Composite Pieces. Some pieces may have a choice of two or more different types of move. In orthodox chess the King and Queen are composite movers.
C5. Compound Moves. A compound move involves two (or more) pieces moving at the same time (i.e. in the same turn of play). The only compound move in orthodox chess is castling, which is a compound move of King and Rook, subject to the conditions that (a) neither King nor Rook may have moved previously in the game, (b) the cells between King and Rook must be vacant. The King is then allowed to move two cells towards the Rook and the Rook occupies the cell moved over by the King. Other conditions related to the royal properties of the King may also come into play (see section E).
D1. Basic Move. The pawn may move only forward. Its basic move is to advance one vacant cell along the file on which it stands.
D2. Option. When on its initial cell it has the option of advancing two cells along the file, provided both cells are vacant.
D3. Capture. The pawn captures by advancing one cell diagonally forward to a cell occupied by an opposing piece, which is removed from the board.
D4. En Passant. A pawn attacking a cell crosed by an enemy pawn which has just been advanced two cells in one move may capture this pawn as though the latter had moved only one cell. This capture may be made only on the move immediately following.
D5. Promotion. On reaching the last rank a pawn must immediately be exchanged, as part of the same move, for a queen, rook, bishop or knight of the same colour as the pawn, at the player's choice.
E1. Royalty. A piece that may not be permitted to be captured by the opponent is termed Royal. In orthodox chess the Kings are royal. If capture of a king cannot be avoided on the next move the game has to end.
E2. Check. A direct atack on a king is called a Check. If a player's King is threatened with capture on the opponent's next move then the king, and sometimes the player, is said to be in Check. If possible, a move must be made that saves the king from check. Pieces that may not be moved into check, or opened to check by the move of another allied piece, may be termed Noble; the difference between noble and royal is that noble pieces may be left in check and captured.
E2. Castling out of Check. The King is not permitted to escape from check by Castling. This rule can be generalised to other variants by saying that compound moves may not be used to save a king from check.
E4. Castling through Check. In Castling the cell pased through by the King must not be under attack. We can generalise this rule for variants to say that a royal piece may not ride through check. There are many examples of chess problem compositions with royal pieces in which the opposite rule is assumed.
E5. Pin. A man blocking a check to the king of its own colour (which is said to be Pinned) can itself give check to the enemy king.
F1. Checkmate. The ultimate purpose of the game is the capture of the opponent's King. Any piece whose capture would end the game is termed more generally a Royal piece. In variants other pieces may be royal, or there may be more than one royal piece. If a player's king is in check, a move must be made that saves the king from check. If no such move is available the situation is known as Checkmate. The game ends and the opponent has won. The situation of Checklock in which the player to move is in check and physicallly has no move available is also normally reckoned to be checkmate.
F2. Other Wins. A player may choose to resign, acceding a win to his opponent. Other situations in which a win can occur without checkmate are when a player exceeds the time limit for his moves, or when a player is disqualified by an arbiter.
F3. Stalemate. A situation in which a player's king is not in check, but all available moves would place the king in check is known as Stalemate, and is counted as a draw. The situation of Deadlock, in which there is physically no move available, is also counted as a special case of stalemate.
F4. Draw by Repetition. A draw may be claimed, and must then be accepted, if the same position is repeated for the third time. A particular situation in which this can occur is Perpetual Check where one player can keep on checking with a cycle of moves.
F5. Other Draws. A draw may be agreed between the players. A draw may also be claimed if fifty moves have been made by each side without a capture or pawn move having been made. Draws may also result from arbitration. (In some special positions it has been shown that a win may be possible after more than fifty moves without capture or pawn move.)
The above treatment of the rules has been put together rather quickly. I hope to complete a more detailed treatment soon.