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

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 ...


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 ...


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 ...


21

What rules (if any) cover a situation like this? 3.10.2 of the FIDE Laws of Chess defines when a move is illegal - 3.10.2 A move is illegal when it fails to meet the relevant requirements of Articles 3.1 – 3.9 Articles 3.1 - 3.9 basically describe the moves of the pieces. Inarkiev's move was illegal because it breached 3.9.2 - 3.9.2 No piece can be ...


18

Here is a well organized list that should validate 99.99%+ of common positions: Board: There are exactly 8 cols The sum of the empty squares and pieces add to 8 for each rank (row) 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 ...


17

It is basically just a shortcut that cuts the game short by one move once the outcome is obvious. According to Wikipedia, "In early Sanskrit chess (c. 500–700) the king could be captured and this ended the game. The Persians (c. 700–800) introduced the idea of warning that the king was under attack (announcing check in modern terminology). This was done to ...


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 ...


15

Capturing the King is an illegal move according to FIDE. Refer to Article 1.2 from the Laws of Chess The objective of each player is to place the opponent’s king ‘under attack’ in such a way that the opponent has no legal move. The player who achieves this goal is said to have ‘checkmated’ the opponent’s king and to have won the game. Leaving one’s ...


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

This is actually very common in blitz games. Taking the opponent's king is considered to be a claim of an illegal move. You can see my answer to this question for some more information. Since taking the opponent's king is a claim instead of a move, it immediately ends the game. In blitz games, the player "capturing" the king wins. In slower games, ...


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 ...


11

So if your opponent promotes a pawn to a Queen and accidentally places your Queen on the board instead of his own, can you claim a win because of an illegal move? No, you can't. There is no illegal move until the player either presses the clock or makes another move (after the opponent has moved). Here is what the FIDE Laws of Chess have to say: 3.7....


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 ...


10

\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.


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

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

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 ...


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

The touch-move rule never compels you to make an illegal move. It may compel you to make a stupid move, but not an illegal one. For example, you'd be forced to move a knight that's pinned against your queen and lose your queen as a result, but you'll never be compelled to move an absolutely pinned knight which would put you in check. See this related ...


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 ...


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