Introduction

This game is inspired by retrograde analysis in chess, in which you start from some position and need to analyze how this position was reached. Such analysis usually includes playing the game backwards, by reverting moves and captures made by each side.

Rules

The game starts with white king on h8 and black king on a1. On each move a player can do either unmove or uncapture. Unmove is a reverting a normal chess move. For example, an unmove of white pawn on e4 could be e4-e2, undoing move e2-e4. Uncapture is undoing a capture. For example, white pawn on e4 can uncapture a black queen by e4-d3(*Q), undoing capture move d3xe4.

A player can uncapture any piece of his/her choice with the following restrictions:

• Uncapture can't result in position with more pieces then available in chess starting position. For example, it is not allowed to uncapture a black queen if there is already a black queen on the board.
• Uncapture can't result in two bishops on the same field color.
• Uncapture can't result in two pawns of same color on the same file.
• Pawns can be uncaptured only on opponent's half of the board and only in front of own pawns on the same file. For example, if white has a pawn on h6, he/she can uncapture a black pawn on h7, but not on h5 or other positions of h-file.

It is allowed to unpromote a pawn, i.e. undo pawn promotion. For example, if white has a queen on g8, he/she can play Qg8-g7P, undoing g7-g8Q promotion move. Unpromoted piece becomes available for uncapture to the opponent.

Uncapture or unmove can't result in opponent's king being in check. If opponent's king is in check and the player to move can't remove this check, he/she loses (so-called retro checkmate). It is allowed to put own king in check after unmove or uncapture, forcing the opponent to remove this check. The first player to uncapture all opponent's pieces wins as well. Draws are possible by agreement, three-fold position repetition or in case 50 moves without uncapture or pawn unmove were made.

ZRF

ZRF for Unplay chess provides posibilty to make unmoves and uncaptures, other rules are not implemented there.

Variants

• Random retro chess. Starting position is randomized: both kings are randomly placed on the board to any legal position.
• Placement retro chess. The game starts in the following way:
• White places his king on any square
• Black places his king
• Game begins (white moves first)

See also

• Retrochess - A chess variant challenge by João Pedro Neto and Ralph Betza.
• Backwards Stalemate Chess by John Kipling Lewis.
• Back to past (discussion about Retro Chess ideas)