The Retrograde Analysis Corner

Miscellaneous Fairy Rules

Glossary

Here we collect short definitions for miscellaneous fairy chess and/or units. They were not found deserving their own special entry (as e.g. Circe chess was). In general, they are only included because we need them for some problem in these pages.

Backward A Posteriori. Backward A Posteriori is a version of the A Posteriori rule for Retractor problems. It stipulates that if any claim pertaining to the right of taking some action is based on certain past happenings, those happenings must be established in the retroplay.
Contact chess. In contact chess, you can only move to a square where at least one adjacent square is occupied.
File rook. A file-rook is a rook that moves along files only (not rows).
Grid-contact chess. Grid-contact chess combines grid-chess and contact chess. You can only move to a square adjacent to an occupied square not in the same 2x2 block.
Plus chess. In addition to its usual powers in standard chess with regard to moving, capturing, or checking from its current square, in Plus chess a man standing on one of the four center squares (d4, d5, e4, and e5) may also capture [but not move without capturing] (and it checks), as if it stood on one of other center squares, provided this square is unoccupied.
Pursuit chess. In Pursuit chess every Black's move must occupy the square that has just been freed by the previous White's move. "Black must pursue White". When several ways of occupying the freed square are possible, Black chooses. When it is not possible to occupy it, Black may play a move of his choice.
Double pursuit chess does exist: there the rule applies to Black's and White's moves.
Neutral units. Neutral units do not belong to any given side: they can be moved by White and Black equally (when they have the move). You can give check with a neutral unit but then the enemy can probably escape the check by moving the checking unit away ! Clearly, you cannot leave your King under the check of a neutral unit.
A neutral Pawn promotes into a neutral officer.
A special form of this is Sentinelles ultra-angevines: all positions must be able to be reached in a game with non-royal kings, also the hypothetical position without bK following the capture threat in a mating position.
Siamese units. A siamese unit is a unit made of two (same nature & same colour) units on two different squares. The pair moves by combining two identical moves of the two components (so that their relative positions remains unchanged). It is necessary that each component move be a legal move by itself (disregarding checks and the other unit). So that e.g. a siamese pawn can only move diagonally if both component pawns can capture an enemy unit. When a siamese pawn reaches the 8th rank, both components are promoted into a same siamese unit. When one component is captured, both disappear.
Rex Multiplex. In Rex Multiplex, you can have several Kings. When mate is in question, all Kings of one side must be mated simultaneously. For retro purposes, additional Kings are promotees. See also Illegal Cluster (Rex Multiplex)
Turbo chess. In Turbo chess every capture by Black must be followed by a double move by White.
Minichess. Minichess is played on a 5x5 board. The begin-position is:
Progressive chess. In progressive chess, every move is a series-move. White starts with a series of 1 move, black answers with a series of 2 moves, white answers with a series of 3 moves, etc.
Alfil The alfil is an old Arabian piece. It moves 2 squares diagonal, the intermediate square may be occupied. For instance, if the bishops in the initial game array were to be replaced by alfils, a legal first move could be Af1-d3.
Camel The camel is a (1,3) hopper. For instance, a camel on b4 can go to a1, c1, e3, e5, c7 and a7.
Transmuting kings If the kings are transmuting, the kings move, when checked, like the checking piece. When not attacked, they move like the standard kings.
Alice chess Alice chess is played on two boards, board A and board B. Moves are done on one board, and the piece is then moved to the same square on the other board. This square must be empty. The king may not put himself in check on either board. At the start of the game, all pieces are on the A-board. In the diagram, upside down pieces are on the B-board, and the normal pieces are on the A-board.
Woozles There are two kinds: mono-woozles and bi-woozles. In mono-woozles, two pieces of the same colour that observe each other can't capture or give check. In bi-woozles, two pieces of opposite colour that observe each other can't capture or give check.
PatCh PatCh, or Patience chess is a fairy variant invented by Noam Livnat. It means that a square which has been vacated cannnot be occupied again for two half-moves in a row. For instance, after 1. e2-e4 e7-e5, white can't play to e2 on his second move.
Exclusive chess In exclusive chess a mate is only legal if it's the only way to give a mate. If white has more possibilities to give mate, none of these mates is legal.
Capture chess Captures are obligatory. There are no royal pieces, one wins by not being able to move anymore (no pieces left, or stalemate) Classical castling rules: King and rook haven't moved, and square the king is on, square he crosses and square he ends on aren't attacked.

Duel chess If possible, the piece that made the last move must move again. I.e. if white starts with 1. Sb1-c3, his next move must be with this knight too. Checks are orthodox (so even if a piece isn't the current duel-piece, it can deliver check). If the piece can't make a legal move, then a new duel-piece can be chosen.

Berkeley chess In Berkeley chess, before a move is made, pieces which aren't controlled by another piece (of either colour) are removed. This removal process is repeated over and over again until no more pieces can be removed. Then a move is made. For instance, in the initial game array, first the rooks are removed, then the knights and the a- and h-pawn of either colour. Only after these removals, white makes his first move.
Crazyhouse Captures pieces become the property of the opponent. They get the colour of that player. Instead of making a normal move, a player may place those captured pieces on the board. Pawns may not be placed on the first or last rank.
Haaner chess In Haaner chess, the departure square of a piece becomes a hole, which can't be played to or passed through anymore. For instance, in the begin position, after b2-b4, there is a hole on b2, which makes a later Bc1-a3 impossible.