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Chess, played backwards

Checkmate Rewind is a detective puzzle built on chess. Every case begins at the end: a game is over, and the final position is the only evidence left on the board. Your job is to find out how the game went by taking the moves back, one at a time, until the pieces stand on their starting squares again.

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How to play

Each case tells you two things: the final position and the exact length of the game in plies. A ply is a single move by one side, so "8 plies" means four moves by White and four by Black. Chess players sometimes call a ply a half-move.

• Move back, not forward. Pick the piece that moved last and drag it to the square it came from. The sides take turns, just as in a normal game, only in reverse.
• Bring captured pieces back. If the last move was a capture, the piece you move back can leave the captured enemy piece on the square it vacates.
• Undo the special moves. Castling is taken back as one move of king and rook. A pawn that promoted can turn back into a pawn on the seventh rank. An en passant capture puts the captured pawn back beside your pawn.
• Every position must be legal. A side that is not to move can never be in check. If an un-move leaves an impossible position, the game will not accept it.
• Hit the number exactly. You must reach the starting position in exactly the given number of plies, not one more and not one fewer. That is what turns a simple rewind into a puzzle.

If you get stuck, take back your last un-move, start the case over, or use a hint. Solve a case without hints or take-backs for three stars.

A first example: Fool's Mate

Imagine a case of 4 plies. White's king is mated by the black queen on h4, and White's f- and g-pawns have moved to f3 and g4. Black moved last, so Black goes first: the queen returns from h4 to d8. Next White un-moves a pawn, then Black's e-pawn goes back from e5 to e7, and finally White's other pawn returns home. The game was 1.f3 e5 2.g4 Qh4#, the fastest checkmate in chess. White's two pawn moves could have come in either order, and both reconstructions solve the case.

Later cases are less forgiving. Pieces go out and come back, a bishop appears on a colour it could not have started on, a king has castled after its rook was captured. To meet the exact count you have to reason about what must have happened, not only about what could have.

Retrograde analysis and proof games

Working out the past of a chess position is called retrograde analysis. It is an old branch of chess problem composition. Instead of asking for the best next move, a retro problem asks what the last moves were, whether castling is still allowed, or which piece was captured where. Raymond Smullyan made the genre famous with The Chess Mysteries of Sherlock Holmes, where the detective solves crimes by reading the board backwards.

A close relative is the proof game: a composer shows a position and asks for a game from the starting position that reaches it in a given number of moves. Checkmate Rewind turns the proof game into something you can play with your hands. You walk the game backwards, the board checks every step for you, and the case is solved when the starting position is restored in exactly the required number of plies.

What is inside

• The Academy: 8 short lessons that teach un-moving, un-capturing, un-castling, en passant and promotion.
• 55 cases in six chapters, from blunders cut off after a few moves to famous opening traps and real puzzles where the obvious answer is wrong.
• A case of the day. On weekdays it follows a real opening line from the Lichess openings database. At weekends it is a real game from Lichess that ended in checkmate. Everyone in the world gets the same case on the same day, and you can share your result.
• The archive: pick any length from 6 to 10 plies and get a new real opening line to restore.
• Investigator's questions. Some cases ask you something about the game, for example on which square a piece was captured. Answer before you restore the game to earn a detective's seal.
• 13 awards, 15 languages, a dark theme, sound, and play on phones, tablets and computers.

Questions

Do I need to be good at chess? No. You need to know how the pieces move. The Academy explains the rest, and the first chapters are short.

Is it free? Yes. There is no sign-up, and your progress is saved in your browser.

Can I play offline? Yes. After your first visit the game opens without an internet connection. On a phone you can add it to your home screen like an app.

Where do the daily games come from? From the open Lichess databases, which are free to use (CC0). Thanks to Lichess and its players.