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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### 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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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 ...
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### 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 &...
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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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I am looking for a good chess website about retrograde analysis problems, but after a few searches, I haven't found anything. Are there any websites that you can recommend?
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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 White piece moved only once?

Here is a chess problem that uses retrograde analysis. I’ve also posted it on the “France-Echecs” chess forum, which is in French of course, and on my blog. [FEN "3qk3/3pp3/8/8/8/3PP3/1PP2PP1/...
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### What’s the difference between Partial Retrograde Analysis and Retro Strategy?

What’s the difference between Partial Retrograde Analysis (PRA) and Retro Strategy (RS) which are different ways of interpreting retrograde analysis chess problems?
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### Challenge: Create the longest set of moves where the game is able to be perfectly reproduced from only the current board state 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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### 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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### Could a chess engine do retro analysis?

If I took a legal chess position, to what extent could an engine work out the previous moves? [in some other board games such as Othello such game reconstruction is done easily with an engine.]
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### Pal Benko's final problem

EDIT: Link added One of my chess heroes, Hungarian GM Pal Benko, is no longer with us. He overcame many difficulties in his early life to live one which was rich and full, and reached the age of 91 ...
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### Identify a problem where a potentially winning move draws because of the 50 move rule

I'm trying to recall a particular chess problem. It's related to retrograde analysis. All what I can remember is this: it's an endgame where there is only one way to reach the position, and during ...
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### Attempting to create a retraction problem

I am trying to create a retraction problem, but I am unsure if it is sound, and I do not know where to find resources on them. I have two questions. Is my problem correct? I am not giving a solution ...
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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 ...
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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'...
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### Searching for a "where is the black king" problem

Sometime ago, a friend showed me a position where he had the Black king removed. The goal was to find where is the Black king using some pretty weird reasoning. There were only a few pieces on the ...
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### Can we say for sure that Black's king has not moved?

The following position appears as the 7th puzzle of Raymond Smullyan's Arabian Knights, a book about retrograde analysis. [FEN "3qk2r/p2pp1bp/bpp2n2/8/3N3Q/1PN1P1P1/P1PPP2P/3K1n2 w - - 0 50"]...
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### What is the result? (Rules + puzzle)

[EDITED to include followup - EDIT 2 to include definition of position] This is in part a genuine question on the FIDE Laws of Chess, and in part a small puzzle to solve. Suppose that during a long ...
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### Can white win in this almost 20 year old puzzle?

It's White to move-can they force a win?. The left bottom is a1 as usual. [FEN "4k2r/6p1/p4pKb/p2N3p/p7/p5Q1/p1n5/qrn5 w - - 9 9"] Note for people who saw an earlier version: the position ...
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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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### Are there any legal positions known to be impossible to come to in a normal game? [duplicate]

There are some fairly obvious ways to set up a chess board that cannot possibly happen: both kings simultaneously in check would be one. There are some positions that seem impossible in a normal ...
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### Which chess position requires the most moves to reach?

For each legal chess position there exist a shortest sequence of moves from the starting position to that position. Which position requires the most moves to reach?
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### Beautiful retrograde analysis problem by Alexsey A. Troitsky

I saw this beautiful, subtle & accessible problem a couple of days ago, and wanted to share it with you folks. (It's by the ground-breaking endgame theorist who e.g. invented the "Troitsky line" ...
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### What was Black's last move?

It's White to move. What was Black's last move, and what could have happened before that? [FEN "8/8/8/8/8/3P2P1/3P2Pk/2KR3N w - - 0 1"]
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