Impossible Checkmate?

Is there a such thing as a position which constitutes a checkmate for one side, but which can't be reached by any legal set of moves.

Let's leave out positions with white pawns on the first row or black pawns on the eighth. ;)

Hrm and let's also leave out positions with more than eight promoted pieces on a side. :D

Another trivial case--the position shouldn't have more than seven pawns on either the a or h files.

If you have one in mind, I'm not sure why you'd refrain from posting it here.

I think I have something else in mind--I'll have to think of a reformulation of the question.

Maybe you mean something like this?

@khpa21:

@pompom: I guess if you add a white rook to the a file then it will work?

But that's basically the same thing as the four queens from before. (You only need two of the queens, I think, btw.) Maybe this is the best kind of example after all.

@einstein: Is it because of the missing black rook? (How could it have gotten out from behind the black pawn line?)

What I'm going to ponder now is the question whether there are positions like this where it takes at least two "reverse moves" to be forced into to a manifestly illegal position. Or proof that there can't be any such checkmate position.

What "manifestly illegal" means as opposed to just "illegal" will be something I'll have to figure out. It may turn out there's no useful such distinction...

That is part of the reason.  TheMouse has the correct explanation in post #12.  You can say that the rook can't get out.  That is true.  But white can still get in there and capture it with a piece.  The main issue is white's pawn structure.

Any position involving a triple check (or more).

You might be interested in a thread like this.  It talks about both legal and illegal positions.  It is mostly positions that are not checkmate though.

This one's actually legal:

Here is one that has checkmate.  There has been some work done in that forum to determine if the position is legal or not.

 I'm stumped. Can you give me a hint?

The last move was a capture.

Black's last move was b7-b5. White's pawn on a5 captured en passant. i think that works, but I am not a great chess player by any stretch of the imagination.

That's it: