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?
1 Answer
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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1Is 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.– supercatMar 13, 2018 at 22:34
pawn+(pawn, bishop, knight), bishop+bishop, knight+knight