51
votes
Accepted
How is this position possible in a normal game of chess?
Double check is only possible by using discovered check. So either the rook check or the bishop check was discovered by moving something in between on the previous move.
I don't see how that's ...
- 26k
40
votes
Accepted
Is the position in this trick question reachable?
Even though the board is upside down, the position is still easily legal.
[FEN ""]
[startply "86"]
[StartFlipped "1"]
1. a4 h5 2. g4 h4 3. Bg2 Rh5 4. gxh5 h3 5. h6 e5 6. ...
- 15.8k
33
votes
Are there any illegal positions that are difficult to spot?
Yes - the less a position looks like a real chess game, the harder it is to spot if it is illegal or not. Sometimes, retrograde analysis is needed to prove a position can be reached in a legal way. ...
- 23.9k
30
votes
Accepted
Given a legal chess position, is there an algorithm that generates a series of moves that lead to it?
The task you are considered is usually called a proof game, named such because the task is to prove that the position is legal. As a genre of puzzles, there are various aesthetic constraints, most ...
- 596
22
votes
Accepted
Prove that White is not allowed to castle here
This is actually a rather typical retrograde problem, just start with the most basic observations:
We see that black is missing both rooks and the f8 bishop. Given black's pawn structure it's easy to ...
- 11.8k
21
votes
Accepted
Least number of moves to achieve a complete switchover of nonpawns in starting setup
As @chakerian's calculations show, 40 moves is the minimum. After a bit of puzzling, I found the solution.
[FEN ""]
1. a4 {First, we need to get the rooks in position. They'll be hard to ...
- 23.9k
20
votes
Can it be proven that 11. 0-0-0+ is legal in this position?
Ok after playing around with a couple of lines, I finally found one line that shows it's still perfectly legal to play long castles for white at move 11, here it is:
[Title "possible 11.O-O-O+"]
[...
- 11.8k
18
votes
Are there any illegal positions that are difficult to spot?
If we define "illegal position" as a position that cannot happen in a game using legal moves, I think that the most difficult to spot would be positions that look normal, but are impossible to achieve....
- 1,198
18
votes
Accepted
Identify a problem where a potentially winning move draws because of the 50 move rule
There are dozens of problems that illustrate a potential winning moves that instead leads to a draw because of the 50 move rule.
One example is the following mate in four published by Léon Loewenton ...
- 713
18
votes
How to solve proof games?
Although solving proofgames is not that difficult, the large number of moves involved may seem impressive at first. Let me share you a few tips by showing how I solve a proofgame.
[Title "Étienne ...
- 713
17
votes
Accepted
Another problem - Find color of the pieces
I'm going to start from scratch even though the OP posted a partial answer in the question, so I will be covering some familiar ground.
I began to break the problem down by assigning black to the ...
- 6,080
16
votes
Accepted
How is this retrograde analysis position legal?
White has made 6 captures. Clearly the h-pawn made at least 2, and the g-pawn 1. Meanwhile, the b-pawn made at least 1 (a7/c7), and the a-pawn at least 2 (to the b-file, then to either a7/c7). So ...
- 818
16
votes
Accepted
Could a chess engine do retro analysis?
Engines like Stockfish and Komodo are not able to work out the previous moves, because that is not what they are programmed for.
However, it is vanishingly unlikely that anybody can ever program an ...
- 4,850
14
votes
Accepted
13
votes
Which methods can be used to prove that a position is illegal?
Easy illegality is easy: not exactly 1 king on both sides, both kings in check, pawns on the final ranks. It's also fairly easy to tally promoted material and subtract the missing pawns. A quick ...
- 10.2k
12
votes
Accepted
"Knight rebus"-Decipher an encoded chess position
Assume the board is in it's usual position (the bottom row is the first row) then this is the solution:
[FEN "1B6/8/1k6/R7/1n6/2K5/r6p/q1r1n3 - - - 0 1"]
and Black's last move was b2xc1R++.
I could ...
- 23.9k
12
votes
What was Black's last move?
There is only one black piece left, so the last move was with the king. Black's last move was obviously Kg1-h2.
The king could not have come from h3, because there is no way the white pawn could have ...
- 20.4k
12
votes
Accepted
Can you mate with each of your 16 pieces on its original square?
Obviously the last white move was 16. Nf3g1+.
I note that black needs at least 15 moves to place the pieces as they are:
4 moves to swap queen with rook as they are (various movements possible),
2 ...
- 20.4k
11
votes
A conditional retrograde analysis problem
The solution is as follows:
11
votes
Accepted
Can we say for sure that Black's king has not moved?
The conditions you gave are incomplete. The conditions include that black (to move) can castle; it's hidden in Smullyan's original problem text.
From the English book, last paragraph of the problem ...
- 4,850
10
votes
Least number of moves to achieve a complete switchover of nonpawns in starting setup
Shouldn't the minimum be 40?. I don't have any result to show yet but it seems that:
Pawns (black and white): 8 moves
Rooks (black and white): 10 moves
Kings (black and white): 7 moves
Queens (...
- 750
10
votes
Accepted
A problem rebus-reproduce the game
Highly non-optimal, but here's one line that reproduces the position. The basic idea is to not capture the existing pieces, but capture pawns. A single pawn capture can free 3 pawns to promote. Here's ...
- 9,235
10
votes
Which chess position requires the most moves to reach?
This was published about in the latest entry of Tim Krabbé's Chess Diary. Harry Goldsteen has put some research into this and came up with a position which is reachable only in 185 moves:
[FEN ""]
[...
- 23.9k
9
votes
Problem: reconstruct the game
This puzzle was by Tibor Orban in 1976, and is a great introductory puzzle for those who have never come across a proof game before. In its miniature form, it still shows two elementary technical ...
- 9,408
9
votes
Prove that White is not allowed to castle here
This problem is by Raymond Smullyan, and is from his The Chess Mysteries of Sherlock Holmes (p.54 of the Hutchinson edition).
Phonon's analysis is mostly sound, except that we don't know which black ...
- 5,570
9
votes
Can you mate with each of your 16 pieces on its original square?
The thoughts mentioned in user1583209's answer more or less summed up mine, but I couldn't find a way to get the king out in time without requiring extra moves. The final trick is
[FEN ""]
1. Nc3 h5 ...
- 23.9k
9
votes
From Mate-in-3 to Mate-in-1
Spotting potential mates isn't too difficult. There are a couple of obvious ones involving the b and h pawns which aren't far off from promoting. An immediate b7 threatens b8=N# for example. It can ...
- 75.7k
9
votes
Given a legal chess position, is there an algorithm that generates a series of moves that lead to it?
If you are familiar with mathematical induction then it should be clear to you that the answer is trivially "Yes".
Just as for any position (legal or otherwise) it is possible to use the ...
- 75.7k
9
votes
9
votes
Find color of the pieces
The difficult part of this puzzle is the realization that the pawn on b7 can actually be black. When this is established, the rest of the puzzle can be figured out without too much difficulty. Thus, ...
- 7,332
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