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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 than 8 Pawns of the same color
  • Having doubled Pawns without the opponent missing any pieces
  • Two Knights giving check at the same time

In the first category, the problem with the position is severe enough that it prevents normal gameplay from continuing (e.g., the victory condition is no longer clear, you might try to capture your opponent's King, and the rules don't define how those Pawns could move). In the second category, while an illegal move must have been made to get there in the first place, nothing would go wrong if you tried to continue a game from there. Chess software seems to make this distinction too: https://lichess.org/editor will gray out "Analysis board" and "Study" for the first category but not the second. Is there an accepted terminology to distinguish between these cases, something like "prograde illegal" vs. "retrograde illegal" or "strongly illegal" vs. "weakly illegal"?

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  • Isn't this question answered by the Laws of Chess? It seems the second category situations are enough for the opponent to stop the clock, and shout for an arbiter, who probably would stop the game as it stands. Or do you think of some other "nothing would go wrong" situations, where Laws of Chess are not involved?
    – user30536
    Feb 1, 2023 at 7:01
  • @user30536 IIUC, both categories' situations are enough to stop the clock and call an arbiter. Feb 1, 2023 at 14:05
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    The term "retroillegal" is in wide use among problemists. And even a problemist wouldn't touch a prograde illegal position without making clear rules how to handle e.g. wayward pawns or multiple kings. For OTB, Brian answers it completely. Feb 2, 2023 at 20:25
  • @HaukeReddmann That term sounds like what I wanted, but I tried and failed to find places that use that term. Can you link to any? Feb 5, 2023 at 20:45
  • @JosephSible-ReinstateMonica: As you surely tried yourself already, epic googlefail (although you surely noticed the 2 positive hits if you combine with "chess" - just to prove existence). This is because you rather find the term in printed media, and since it is sort of a shorthand, not necessarily even there if it fits. Feb 6, 2023 at 7:35

1 Answer 1

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As far as the FIDE Laws of Chess are concerned there is only one type of illegal position and that is:

3.10.3 A position is illegal when it cannot have been reached by any series of legal moves.

There is no distinction between the 6 types you give. In all cases a legal game of chess cannot be played.

That said, the Laws do give two different solutions for when that occurs, one when the moves are recorded and one when they are not.

In the case that the moves are recorded the first illegal move can be found and the position restored to that immediately prior before play continues in accordance with 7.5.1:

7.5.1 ... 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.

In the case that the moves are not being recorded, i.e. rapid or blitz, then the principle in the rapid rules, A.5.4, should be applied. That is the arbiter should wait for one more move to be played and then either declare the game drawn, if the position is still illegal, or allow play to continue if the illegality has resolved itself.

The arbiter should probably also refrain from explaining this before implementing it unless specifically asked since explaining could give the player on move an advantage. If the player on move is losing then it is in their interest to make a move which preserves the illegality of the position and get a draw and vice versa if the player stands better. They should strive to play a move which returns the game to a legal position if possible whilst retaining winning chances.

A.5.4 If the arbiter observes both kings are in check, or a pawn stands on the rank furthest from its starting position, he/she shall wait until the next move is completed. Then, if an illegal position is still on the board, he/she shall declare the game drawn.

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