Object: Retro Problems (Unplay the given situation back to the starting position).

This game plays chess moves backwards only.

You are given a chess position.
(When you create your own setup, you must also update piece- and pawn-counters at the right border).
Play chessmoves backwards (e.g. Pawn e5-e4). You win when you reach the standard chess starting position.
Usually you have to move the piece concerned (and not just click the target square).
Then select one move from the list of moves offered (if such a list is offered).

Typically you might play this game as a 'solitaire game', playing for both White and Black.
However, it is equally possible to alternate between two players and play this game cooperatively.
Of course only White can 'win' this game.
The challenge is in finding all the right moves to actually end up with the standard chess starting position.

Before the unplaying starts, click one of the three buttons at the lower right border to tell the system whether you want to have all help possible / warnings only / loss messages when you made an illegal move.
Mode 'All help possible' (strongly recommended): the system tries to offer you only legal reverse-chess moves.
You lose (or get a warning) if after your move your opponent is in check (the system usually does not offer you such moves anyway, but it does not detect these cases before the move has been executed, especially in capturing moves).
You also lose (or get a warning) when you move your king into an impossible double check (however, not all such impossible double checks are detected by the system).
You also lose (or get a warning) after a move which results in a player possessing more than 16 pieces or more than 8 pawns.

Click the PASS button if Black is supposed to start the retro sequence.
(Note that in some retro-problems it is not clear whose turn it is!)

Variant 1: Standard setup for your own retro problems. White starts. Right click the mouse to set up your own starting positions.
Variant 2: Standard setup for your own retro problems. Black starts.
Variants 3, 4, 5 : Retro problems by Karl Scherer Solutions attached. Proofs of uniqueness attached in text form in directory 'Proofs'.
Variant 6: Retro problem 4 by Harry Nelson and Karl Scherer. Question: can White still castle? In other words: try to unplay the game without ever moving the white Rooks or the white King.

Moreover:
- 2 Minimum-attack problems (see associated game texts).
- 2 Straightjacket Chess problems (see associated game texts).
- 6 Masterworks of retro analysis (according to Fabel, 'Die Schwalbe' 1985).

The idea for this game 'Unplay Chess' stems from the chess problemist area of retrogade analysis.
This area comprises many types of chess problems where for the solution some previous moves have to be considered:
- Mate in ... moves
- Can White (Black) still castle, and on what side?
- Is there an enpassant move possible this situation?
- Find the last ... moves.