This position was reached in a normal chess game:

                   BLACK
     +---+---+---+---+---+---+---+---+
     | B |///| K |/R/| N |///|   |///|  
     +---+---+---+---+---+---+---+---+
     |/P/| P |///|   |///|   |///|   |  
     +---+---+---+---+---+---+---+---+
     |   |///| Q |/K/|   |/R/|   |///|   
     +---+---+---+---+---+---+---+---+
     |///|   |///|   |///|   |///|   |
     +---+---+---+---+---+---+---+---+
     |   |///|   |///|   |///|   |///|
     +---+---+---+---+---+---+---+---+
     |///|   |///|   |///|   |///|   |
     +---+---+---+---+---+---+---+---+
     |   |///|   |///|   |///|   |///|
     +---+---+---+---+---+---+---+---+
     |///|   |///|   |///|   |///|   |
     +---+---+---+---+---+---+---+---+
                   WHITE

a) Identify the color of each piece.
b) What was the last move?

The notation here is a little confusing.  He's using the slashes here only to identify the colors of the squares, not anything about the pieces.  So the two pawns and the bishop aren't so mysterious.  The bishop could be black and both pawns could be white, for example, or various other combinations.

The mysterious part is with the rectangle formed by the kings, the queen, and the D8 rook.  The queen and rook can't be opposite colors, as that would put both sides in check at the same time, which is impossible.

So the queen and D8 rook are the same color, which raises the tricky question of how they both managed to put the enemy king in check (whichever king is the enemy).  You can't just move them into place; if one of the two put the enemy king into check, then the king must move out of check before the second piece can be brought to bear.

The only solution I see is for the D8 rook to have been promoted from a capturing pawn at C7.  That would make the C8 king the enemy, and he wasn't in check the last time his side moved.

From this, we know that the B7 pawn can't be white if the C8 king is black, as this would create another pre-existing check problem.  Also, the F6 rook and E8 knight must be the same color as the queen and D6 king, or that side would have had to move out of check.

The B7 pawn can't be black, or it would have never moved, meaning that you couldn't have a bishop at A8.  So the B7 pawn is white.  This means that the C8 king is also white, because otherwise it would have a pre-existing check problem.

Now that we know that the C8 king is white, we know that the other king, the queen, both rooks and the knight are all black.

But I don't know how you could determine the colors of the bishop and pawn at A8 and A7.

And for what it's worth, the white C8 king isn't yet in checkmate.  He can capture the rook at D8, then the queen will mate him by moving to D7.

Posted by Leonidas on 2006-01-10 20:02:04