The title says it. Reading up on how to describe the position of a board, all of the requirements make sense (like two kings not being on adjacent squares and so on) except for the active color being in check. Why isn't that allowed? What is the rationale behind this?

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    I'm not sure what you intend with the phrase "active color", but I would guess you mean the side which currently has the move. Is that right? Because if so, it's a requirement that the side without the move not be in check in order to have a legal position; the side with the move can of course be in check. – ETD Nov 13 '12 at 5:48
  • I wrote something about this, there are two rules 1)non-active is not in check, 2)active is checked less than 3 times, and in the case of two checks, it must never be pawn+(pawn, bishop, knight), bishop+bishop, knight+knight – ajax333221 Nov 13 '12 at 17:26

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 King to remain in or be exposed to check.

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    Is there any particular benefit to limiting FEN to positions that would be reachable via sequence of legal moves? Having a common notation for chess variants that use the same pieces but may have different rules, or for composed puzzles that may not be reachable via normal play, would seem more useful than one which is more restricted. – supercat Mar 13 '18 at 22:34

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