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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### 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 ...
141 views

### 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.
7k views

### 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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### What are the castling & e.p. conventions, and how are they used?

Castling & en passant are often called "conditional moves". In the chess problem world, we frequently hear about the castling & e.p. conventions. What are these exactly, and how are ...
264 views

### 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 ...
182 views

### 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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### 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 ...
53 views

### 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 ...
439 views

### 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 ...
1 vote
71 views

### 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 ...
1 vote
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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 ...
264 views

### 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 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 ...
920 views

### 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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### 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 ...
400 views

### 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? ...
346 views

### 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 ...
108 views

### 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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### 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 ...
1 vote
76 views

### 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, ...
114 views

### 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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### 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 ...
178 views

### 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. ...
390 views

### Seeking a particular problem

I recall seeing a problem in Games magazine, probably in the early 1990s, that required the solver to deduce that a capture en passant was possible (not just because that's the only way to get a ...
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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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### Is there fiction dealing with retrograde chess analysis?

I am interested in retrograde analysis problems, where the question usually is: 'What was the last move made?' or 'Where did the piece stand on the board before it fell from the table?' etc. ...
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### Challenge: Create the longest game determined only by the final diagram and move number

The challenge is to create the longest list of half moves whereby an AI could re-create that list of moves perfectly, only given the ending board state and the number of moves. As example I will first ...
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### Are there any illegal positions that are difficult to spot?

I have a question and I'm sorry if it sounds very uninformed, I'm not a professional chess player so my interest is mere curiosity. I was wondering if there are examples of illegal positions that ...
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### A question regarding "The Horse Concoction"

Long ago in 2000, Tim Krabbe presented, a now classic, problem by Harry Goldsteen in his article “The Horse Concoction” on his site. It has blossomed across the Internet, as some of us know. The task'...
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 ...
656 views

### Mate in One the Missing King

The post "A mysterious mate in one" made me think of one of my last problems which is also a Mate-in-One with no black King: [Title "White to move and mate in one"] [FEN &...
134 views

### 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... / ...
303 views

### 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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### 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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### 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 ...
1k views

### Retrograde analysis of a position in "The Flanders Panel"

Has anybody tried to verify the retrograde analysis of a game from the book "The Flanders Panel" by Pérez-Reverte? So far, I have found only a board reconstruction in a post on The ...
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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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### 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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### A mysterious mate in one

Here's a question that's a little off the usual format, but problem questions like this have been done before and are on topic, so here we go! Question: Can you find the mysterious mate in one? The ...
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### Where are the non captured Black pieces?

[FEN "8/8/8/8/8/PP4PP/2PPPP2/NNBQKBRR w - - 0 1"] Here is a problem from my blog once more. This is the position after White's 14th move. Seven Black pieces were captured during the game ...
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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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### 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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### 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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### 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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### Another problem - Find color of the pieces

Please help me to find the solution of this next problem by the legendary composer Andrej Kornilow. It appeared in Shakhmatnaya Kompozitsiya 41 issue in 2001 and it was dedicated to A. Lobusow on his ...
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### Can you mate with each of your 16 pieces on its original square?

An opinion poll conducted by problem composer Roberto Osorio over 10 master-level chess players in the Argentine Chess Club produced 100% wrong answers! :-) When the right answer is posted here, I ...
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### Where can I find these mentioned chess problems?

In his article “The Horse Concoction", Tim Krabbe mentions retrograde analysis problems that involve placing 10 rooks, knights, or bishops on the board for a mate in 1. The 10 rooks problem shown ...