Retrograde Analysis/Examples/Example 1

Example of Retrograde Analysis
Consider the following chess position:

Move $-1$
Black is in check from the bishop on $\text a 1$.

Thus move $-1$ must have been white to place black in check.

That must have been done by moving something out of the way from between the black king on $\text f 6$ and the white bishop on $\text a 1$.

That piece cannot have been one of the following:
 * the bishop on $\text g 4$ (as it is in the wrong place)
 * the king on $\text g 8$ (as it is in the wrong place)
 * the queen now on $\text e 2$ (black would have been in check at the end of the last move, which is illegal)

Hence the last piece to move must have been the pawn now on $\text e 6$.

It remains to determine where $\text e 6$ came from.

It is noted that $\text e 6$ blocks a check on white from the bishop on $\text a 2$.

If $\text e 6$ came from $\text e 5$, white would have been in check before his last move.

But there is no legal move from black that could have resulted in white being in check if he had not been before.

So $\text e 6$ must have made a capture from either $\text f 5$ or from $\text d 5$.

However, this cannot have been a normal pawn capture, as that would mean there was nothing blocking black from being in check from $\text a 1$ at the end of move $-2$.

Hence move $-1$ must have been an en passant pawn capture of a pawn which must have moved from $\text e 7$ to $\text e 5$ on move $-2$.

$\text e 6$ must have started on $\text d 5$ otherwise white would have been in check at the end of move $-3$, which is illegal.

Hence move $-1$ must have been:


 * $\text d 5 \times \text e 6 \text { (e.p.)}$

Move $-2$
In order for the en passant capture to be legal, the last move must have been:
 * $\text e 7 - \text e 5$

Move $-3$
At the end of this move, black is in check from $\text a 1$.

So white must have put black in check.

This can have happened only by a white piece being moved off the $\text a 1$ -- $\text f 6$ diagonal.

That piece cannot have been the queen now on $\text e 2$, because black would have been in check at the end of the move $-4$, which is illegal.

Hence it can only have been the pawn now on $\text d 5$, which must have been on $\text d 4$.