42

Yes. Besides making sense, it's also explicitly stated in the rules of the game: 3.9 The king is said to be 'in check' if it is attacked by one or more of the opponent's pieces, even if such pieces are constrained from moving to that square because they would then leave or place their own king in check. No piece can be moved that will either expose the king ...


30

Can a piece put a king in check even though moving that piece would be an illegal move? Yes. One way to see why this makes sense is to imagine "checkless chess", a game that is just like chess, but you win by capturing the king, not by delivering checkmate, and no one is forced to move out of check. This game is exactly the same as chess except that it ...


30

This only really applies to over the board games (since online chess servers prevent illegal moves). In a standard time control game, under USCF rules, when the illegal move is noticed by one of the players, a few things happen. First, the illegal move must have been made within the last 10 moves. If not, then the current position stands, and play ...


29

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. For an example, see the starting position of the Horse Concoction, which can be proven to be legal: [FEN "8/7P/1P5B/2B1Q1n1/3nn2P/1PRnk1nR/3nnnK1/2B1nQBn w - - 0 ...


21

One rationale that I can think of for not allowing the king to move through check parallels that behind the possibility of capturing en passant after a pawn makes a two-square advance. The typical pawn move is just a single square forward, and the possibility of advancing two squares on a pawn's first move was a relatively late addition to the game in ...


17

Here is a well organized list that should validate 99.99%+ of common positions: Board: There are exactly 8 cols The row sum of the empty squares and pieces add to 8 There are no consecutive numbers for empty squares Kings: See if there is exactly one w_king and one b_king Make sure kings are separated 1 square apart Checks: Non-active color is not in ...


16

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. [FEN "2k1rr2/pbpq2b1/1p1p1np1/nB1Pp2p/2P1Pp2/2N2NP1/PPQB1PP1/2KR3R w KQkq - 0 1"] For example this position looks legal, but indeed there is no way how this ...


13

The way I always understood castling is that it allows the player to move his king to safety. But this privilege does not come for free - it comes at the cost of a tempo. If a player was allowed to castle out of check, or over a checked square, then it allows him to postpone this powerful move until the very latest, effectively removing the penalty from the ...


12

In standard play (each player has at least 60 minutes for all the moves) it makes no difference when the illegal moves is spotted the position must be restored to the one before the illegal move even if that was several moves ago. In rapid or blitz Appendix A4 part 2b applies: b. An illegal move is completed once the player has pressed his clock. If ...


11

From your description, I'm not sure whether in your childhood games the sort of double move you showed above could only happen at that one point in the game, or could continue to happen. So I don't have much to say, but there is at least an established chess variant (dating back to the early 20th century) in which your indicated sequence of moves could occur ...


11

Castling is the only legal chess move in which two pieces are moved. You can verify this by searching for "FIDE Laws of Chess" and reading "Article 3: The moves of the pieces" (I do not provide the URL here because unfortunately it frequently changes).


11

A player who makes an illegal move must retract that move and make a legal move. That move must be made with the same piece if possible, because the touch-move rule applies. If the illegal move was an attempt to castle, the touch-move rule applies to the king but not to the rook. The arbiter should adjust the time on the clock according to the best ...


11

Hi David and welcome to chess.stackexchange.com. This is a common question. It's illegal to move a king into check, or to leave it in check. This includes moving a king adjacent to the opposing king. If you have no legal moves then the game is over: if your king is in check then you are checkmated and you have lost, otherwise you are stalemated and the ...


11

FIDE Laws of Chess 7.4a If during a game it is found that an illegal move, ..., has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined the game shall continue from the last identifiable position prior to the irregularity. The clocks shall be ...


10

I just posted an answer to this question How did castling originate? and I think it offers a missing part of the answer to this questions as well: What the rationale is, behind not being able to castle out of check. Historically castling was probably two moves (the rook move and the king's leap), that were merged into a double move, because they basically ...


9

The FIDE rules say this: 7.5 If during a game it is found that an illegal move has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined, the game shall continue from the last identifiable position prior to the irregularity. Articles 4.3 and ...


8

\s*([rnbqkpRNBQKP1-8]+\/){7}([rnbqkpRNBQKP1-8]+)\s[bw-]\s(([a-hkqA-HKQ]{1,4})|(-))\s(([a-h][36])|(-))\s\d+\s\d+\s* Here's a regular expression that I use to ensure that a FEN string is actually valid. It doesn't do any testing for a legal/illegal position, but it's a good starting point.


8

A piece can give check even when it is pinned. This is the main "exception" to the rule that a pinned piece cannot move. The reason is, your pinned piece giving check "takes" the opposing king first. (In this case, it's your rook at g2 on the g file.) That's BEFORE his bishop would take your king. So by the laws of the game, your friend had to move his ...


8

Yes, that's legal. For example: [FEN "6k1/6r1/8/8/8/6R1/8/6K1 w - - 0 1"] [SetUp "1"] [CurrentPosition "6k1/6r1/8/8/8/8/6R1/6K1 b - - 1 1"] 1.Rg2 The rook can move along the g-file. This is known as a "partial pin" - the rook is somewhat limited in its movement (and can't go to a3, for example) but it can still move if it still protects the king.


7

It is never, ever legal to move into check. Period. No ifs, no buts. No exceptions whatsoever. That's the best way to think about it, since it gives the least scope for confusion. If you want to think of it in terms of capturing the king ending the game, look at it this way: if you move into check, your opponent can capture your king, which immediately ends ...


7

It's not technically a check since Black wasn't making a move to check White's king. White made an illegal move that placed his King under attack. Black should mention that White made an illegal move, getting White to take the move back and play something else. If you're playing in a tournament, you could call the arbiter over and say White made an illegal ...


6

From FIDE's Laws of Chess 7.5 a. If during a game it is found that an illegal move has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined, the game shall continue from the last identifiable position prior to the irregularity. Articles 4.3 ...


6

Black attempts to claim a win under 7.5.5. Does this stand? No. According to 7.5.5 - After the action taken under Article 7.5.1, 7.5.2, 7.5.3 or 7.5.4 for the first completed illegal move by a player, the arbiter shall give two minutes extra time to his opponent; for the second completed illegal move by the same player the arbiter shall declare the ...


6

The player of the white pieces has made an illegal move. Touch-move applies so he must take back his illegal move and make a legal move. If it is possible to make a legal move with the piece he touched then he must make a legal move with the touched piece. If there are no legal moves with the piece he touched then he may make any legal move. This is ...


6

pgn-extract will do exactly that if you give it an input file and say where you want the correct games to go. For instance: pgn-extract --output clean.pgn games.pgn will put only the valid games from games.pgn into clean.pgn, reporting any errors on standard output. If you want to have a record of the errors then you can use the -l flag to specify a log ...


5

The active color (i.e. the color which has the next move) is allowed to be in check, and as ajax points out, at most twice. But perhaps you meant the inactive color not being allowed to be in check. Why is that? The answer is: if the inactive color is in check, it would mean that at least the last move by the inactive color was illegal, allowing its own ...


5

PGN files are normal text files. You can generate it to somepoint using the software and then save the file, open it in a text editor (notepad, notepad++, ...) and continue editing there.


5

The final position in this sequence is legal. (See Baibikov, "Length records in 'Last single moves?' problems", A15.) It is remarkable in being the lightest-known position in which the last 17 single-moves can be determined, neither king is in check and it is not specified whose move it is. It can be deduced from the board position that White moved last. ...


5

This is an illegal move. Even if the White King is protected, you can never put your King en prise. However, if you were to play such a move and your opponent didn't call it ON THAT MOVE, an interesting scenario arises. There was a Carlsen - Inarkiev blitz game where Inarkiev made an illegal move, and then Carlsen made an illegal move. Inarkiev tried to ...


5

The FIDE laws of chess state: If during a game it is found that an illegal move, including failing to meet the requirements of the promotion of a pawn or capturing the opponent’s king, has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined ...


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