# Questions tagged [retrograde-analysis]

Retrograde analysis is the method of deducing information by working backwards from a given position or outcome. This information can be about past positions of the same game, or may contain forward content, such as optimal play.

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### Can a series of legal moves end in this position?

With white having all pawns promoted to queens, and all pieces in their original positions, and black only having their king left, checkmated?
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### What's the answer to this castling rights problem?

[Title "#2 - Leonid M. Borodatow. Die Schwalbe 163 02/1997"] [FEN "r3k3/2pN3P/5PPP/6pp/7p/6P1/N5P1/R3KB2 w - - 0 1"] Different sources give different answers.
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### Long proof game with rooks

I am trying to track a very special proof game I saw many years ago when I learned about proof games (a board position is given, and the challenge is to find the unique sequence of moves from the ...
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### Does this problem work under the standard AP condition?

[Title "André Hazebrouck. Die Schwalbe 47 10/1977 4. Preis"] [FEN "r1b1k2r/P2p4/2p1n2p/ppPq4/1np5/p7/P2PP3/R3K2R w - - 0 1"] а) h#2,5 b) h#2,5 (AP) 2 solutions What is meant by ...
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### How does this problem relate to the 50 & 75 move rules?

[Title "Draw - T. Volet. Ded. M. Caillaud & N. Plaksin. 1st Prize Rex Multiplex, 1983"] [FEN "n4K2/b1pprppp/1p2p3/2p5/N1P3P1/BkP1P3/1P2PPP1/3N1BR1 w - - 0 1"] I have an ...
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### Is the RS method actually a convention? If so, what can be written in it that is not in 16.1 and 16.2?

The RS Agreement is mentioned in paragraph 16.3 of the Code. But in such a context that I want to cry. ... then the Retro-Strategy (RS) convention should be applied: whichever castling is executed ...
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### What is the correct retro protocol for this problem? PRA or RS?

[Title "#1 - W. Keym. Die Schwalbe, 1969. Ded. K. Fabel"] [FEN "qrbR4/rP1P1pp1/pQ1pp3/pPp5/6P1/1P3BB1/3PP2P/R3K1k1 w - - 0 1"] The solution to the problem is simple. Black has ...
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### Does the AP Convention and Rule 16.2 allow E.P. to be performed without justification?

Rule 16.2 En-passant convention. An en-passant capture on the first move is permitted only if it can be proved that the last move was the double step of the pawn which is to be captured The AP ...
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### Position where last x halfmoves are determined

What is the largest integer x such that there exists a position such that: That position can occur in a legal chess game It is possible to prove at least x halfmoves have been played It is possible ...
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### What conditions must/should be added to this fairy selfmate stipulation?

[Title "s#2 (?) Maximummer - Anatolii Vasylenko Die Schwalbe 132 12/1991 4. Lob"] [FEN "1K2k2r/r4R2/8/QPp4p/8/8/8/8 w - - 0 1"] 1. bxc6ep Rxf7 2. Qxh5 0-0# 1. ... ...
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### What is A Posteriori (AP)?

A number of problem-related questions recently mention AP by which they mean “A Posteriori”. What are they talking about? It seems as if the guys talking about it don’t have a common understanding. Is ...
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### Can a unique position be deduced if pieces are replaced by checkers (can see piece color but not type)

Inspired by a video where top players guess which famous game a position is from, except pieces in the position are replaced by checker pieces (so they can see the piece color but not type). Of course ...
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### Show an AP Type Keym #n, where Black's castling illegality with WTM drives both basic White mate & AP?

Let me explain with an example what is meant. [Title "#2 - Lord Dunsany. Fairy Chess Review, 1943"] [FEN "r3k3/ppp5/2p2P2/3PpK2/8/NP1R4/p1P3P1/1b3N1R w - - 0 1"] 1. Ke6 (0-0? ...
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### Why is the abbreviated retronotation not used?

When playing forward, both full and abbreviated notation are used with equal success. Decoding the abbreviated notation does not cause any problems for the user. Why can't you do the same in retro ...
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### Is Retro Zugzwang possible?

What is just a zugzwang is clear. This is when, in a normal good position, a side is forced to make a move, and any move made leads to a loss. Accordingly, in Retro moves are made not forward, but ...
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### Can the problem condition "A Posteriori Type Keym", which flips the player to move, also flip the player that delivers mate?

Again, I composed the task, but doubts crept in. Is everything really as I think it is? 4k2r/4p3/4P3/7N/8/P7/rP1PP1P1/5KQR w - - 1 13 2b1k2r/1p1pp1p1/4P3/p6N/6P1/2P5/1PrPP1P1/1B3KQR w - - 1 13 ...
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### Have problems similar to this recent composition of mine been done before?

N1Nrk2r/n1ppp1p1/1pQ5/1P6/5R1P/3P3B/3PPBPp/R3K3 w - - 1 13 Checkmate in two moves. AP (Type Keym + Type 3) The AP Convention (A Posteriori) states: "If the player needs castling rights in order ...
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1 vote
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### Lower bound on the number of moves to reach given position

I'm trying to analyze opening positions using the Lichess evaluations database. Loading the entire database into memory is somewhat costly, so I'd like to filter out positions that occur after, say, ...
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### Do endgame tablebases only handle legal positions?

The title is my question for now. I also wonder if similar legality concerns may reduce the number of input FENs as checkmates used as input to the retrograde proofing or construction process. In ...
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### Max number of promoted units on the board in a monochromatic game

A whimsical question just occurred to me. I think I have an answer, but I thought I would share it here as a puzzle. What is the maximum number of promoted units that can appear simultaneously on the ...
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### What are "monochromatic" retrograde analysis problems?

What does it mean when a retrograde analysis problem is qualified as being "monochromatic" ?
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### Missing move in a chess match due to lousy annotation

I am a beginner in championships (well, and in chess in general). The other day I played a match that I can't analyze because there is an error in the annotation sheet. [FEN ""] 1. e4 e5 2. ...
1 vote
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### Could castling ever be proven to be legally allowed for sure if the first move is required to be a pawn move?

In retrograde analysis problems, castling is assumed to be allowed unless it could be proven that either the king or the rook has previously moved. It is well known, of course, that one could never be ...
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### How would I detect dead position in a tablebase?

I am programming a tablebase for fairy chess, and would like to conclude the game if mate cannot be reached in any way. I guess this has to be detected when the evaluation (win in... / loss in... / ...
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### When can you tell which pieces are 'White' from a position where the pieces are colored red and green and you know whose move it is? [duplicate]

Suppose you come across a chessboard where the pieces are colored red and green and there are no labels on the squares. You don't know which move number it is. What are the positions where, by only ...
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### Term to distinguish between two categories of illegal positions

Consider these two categories of illegal positions. First, e.g.: Not having exactly one King of each color Both Kings being in check Pawns on the first or eighth ranks Second, e.g.: Having more ...
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### Legal positions with one color to move only?

Most positions in chess can be reached with both black or white to move. Some positions are only legal with one color to move. One such position is the starting position, which famously can't be ...
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### Do any unique proof games feature an en-passant capture?

Most positions that could be reached, in a specified number of moves, after an en passant capture could be reached just as well without one, suggesting that such positions could not occur in unique ...
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### Open problems in chess involving maximum material imbalances

I am posting the present question since here Very unbalanced Chess Positions I have introduced three open problems (i.e., the third one, the fourth one, and the sixth one from the above), regarding ...
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### How many more squares can you attack?

(Inspired by this cool problem) Let’s say it’s White to move. Let A = the number of squares currently attacked by White, and B = the number of squares attacked by White after White’s next move. What ...
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### Which piece was captured last - knowing that this position was reached in the minimum number of moves possible?

This is a puzzle that I made a while ago, but never really published anywhere before (although I posted it on an obscure Dutch telegram channel). The question is simple: what was the piece that was ...
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### Which methods can be used to prove that a position is illegal?

Given a board position, there are programs that try to find a series of legal moves that lead to it. Are there algorithms that can do the opposite - prove that the position is not reachable by a ...
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### How to solve proof games?

The following question deserves to be asked! "I see a lot of proof game problems, where I am given a diagram and have to find the unique way to reach that position in the specified number of ...
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### Generate an entire chess game up to a position

I recently found a chess problem YouTube video with this position. [FEN "r4rq1/5p1p/p5kn/R1B1Q1b1/6p1/5p2/PNP2P2/1K6 w - - 0 1"] I want to turn this position into an entire game of chess. ...
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### Reach this position in four moves [duplicate]

I recently came across this chess problem. The requirement is to reach this position (starting from a basic board set up board) in 4 moves. How do you do it? [FEN "rnbqkbnr/pp3ppp/2p1p3/8/4P3/8/...
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### Mate in 1, then No Mate in 1, then Mate in 1

Anand and Bobby both are members of their school's chess club. And while cleaning up the games room at the end of the day, they saw a board with the following position: [FEN "rbR1B1Q1/k1n1nN1p/...
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### Is the position in this trick question reachable?

I found this trick question on r/Anarchy chess: White to move, mate in two. [FEN "2R2RK/6PP/5P/3PN/P/1pQ3p/n1p1qbp/5brk w - - 0 1"] [StartFlipped "1"] The trick is that But is ...
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### Harry Potter Chess Analysis

So, I've been reading Harry Potter and The Philosopher's Stone. If you haven't read the book yet, or watched the movie, and you don't want any spoilers, stop reading this question. Continuing, there ...
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### Coordinates probem

[FEN "8/8/8/8/8/4R3/2N2R2/3N1k1K w - - 0 1"] Given that this position could be reached in a legal game of chess, and that no piece was captured on the last move, are we seeing the board ...
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### Where does the black king stand in this specific position?

[FEN "7r/pppppKpP/6P1/8/8/8/n7/8 w - - 0 1"] Assume that the following position was reached in a legal game of chess. In the last 5 moves, no pawn has moved and no pieces were captured. The ...
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### Brainteaser puzzle

So... initial position is [FEN "7k/8/6KB/8/p7/8/Q7/8 w - - 0 1"] Now... Neither white's king nor queen has moved in the previous five moves. In addition, no pieces have been captured by ...
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### Find color of the pieces

Given the following position, what colour are the pieces and what was the last move played? [FEN "B1KRN3/PP6/2QK1R2/8/8/8/8/8 w - - 0 1"]
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### Do you know this proof game?

I'm not familiar with this proof game. [FEN "r2qk1nr/ppp1pp1p/2n1p2p/8/8/1P6/NPPPPPPP/R3K2R w KQkq - 0 1"] What square was the white queen captured and how?
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### Retrograde analysis problem

Given that this position is legal, and could be reached in a legal game of chess, what were the two last half-movess, or one move for White and one for Black? [FEN "5K2/8/6Pk/8/5Q1R/8/8/8 w - - 0 ...
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### Given a legal chess position, is there an algorithm that generates a series of moves that lead to it?

Given a FEN board position, is there an algorithm that can return a PGN move list that evaluates to it from the standard initial position?
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### Help needed with a problem

On an empty board, how can you place a White king, two White, rooks, and a Black king so that White can give mate in 4 ways from that position?
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### How many moves needed to reach this position?

"Homebase" is where all the surviving pieces are apparently on their starting squares. Here is a very easy new example proof game: can you find the unique shortest game that leads to this ...
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### From Mate-in-3 to Mate-in-1

Here is my new problem in 3 steps - the key designates here the piece that moves first in the solution. An other way to reach a mate in one I hope you'll enjoy... [Title "From Mate in 3 to ...