The Simple and the Complex

One of the standard techniques to understand an endgame is to introduce small changes into the position, then determine how the changes affect the initial evaluation. I applied this technique to a position I discovered in a recent post on Drawn Rook Endgames Despite a Two Pawn Advantage (the reference no.2378 is explained in that post).

The position on the left is a draw. According to the tablebase, Black's only move to achieve it is 1...Rf2+. Now if 2.Ke6, 2...Rg2 wins the g-Pawn, leaving an elementary Rook and Pawn draw. White to move in the diagram wins with 1.e5

The same start position, shifted one file to the left is shown in the right diagram. I expected this to be a draw also, but White wins whichever side is on move. With Black to move, 1...Re2+, echoing the solution in the first diagram, fails to 2.Kd6. The try 2...Rf2 loses to many moves, of which 3.d5 gives the fastest win.

At first I was baffled by this result, but then realized that the reason is simple. In the first diagram, the surviving Pawn is on the e-file. After eliminating the g-Pawn, the Black King ends up on the 'short side' of the Pawn (the three files to the right of the e-Pawn), while the Black Rook is on the 'long side' (the four files to the left). The Rook has sufficient space to harrass the White King with checks from the side.

In the second diagram, the surviving Pawn is on the d-file. This time the Black King ends up on the 'long side', while the Black Rook is on the 'short side', with insufficient space to harrass the White King.

Chess can be so simple and so complex at the same time. This is even more true of the endgame than of the other phases.