Endgame: Bishops of opposite color

Continuing with Capablanca's games 'to be studied', the diagram shows another position from Endgame: Nimzowitsch - Capablanca, Riga 1913. That second link points to the full game and has more about the references mentioned here.

Capablanca played the natural 29...Bxg4, to which Nimzowitsch replied 30.a4. In 'Basic Chess Endings', Fine gave a long variation starting 30.Ke1, showing that it also lost. This is worth a separate analysis that I didn't do.

Riga 1913
Capablanca, Jose Raul

Nimzowitsch, Aron
(After 28.Kg1-f1(xR))
[FEN "8/pp3k2/7p/2p1B1p1/6P1/2P2b2/P1P4P/5K2 b - - 0 29"]

The game continued 30...Ke6 31.Bb8, and now Capablanca played 31...a5!. This move is one reason the endgame has been included in so many anthologies.

The first point of Black's 31st move is that after 32.Bc7, Black has 32...b5!. Fine gives the sample variation 33.axb5 a4 34.c4 a3 35.Ba5 a2 36.Bc3 Kd6 and 37...Be6. After capturing the c- and b-Pawns, Black will have an extra Pawn on both sides of the board, which is sufficient to win an endgame with Bishops of opposite color (BOC). In fact, in any BOC endgame with B+2P vs. B, where the Pawns are disconnected, the Pawns must be separated by at least two files to have winning chances. One file is usually not sufficient to win when the enemy King is in front of the Pawns.

White saw this and played 32.Ke1. Now after 32...Kd5 33.Kd2, comes the second point of 31...a5!. White can't go after the Pawn with 33.Bc7, because of 33...Kc6 34.Bxa5? b6, trapping the Bishop.

Here Capablanca noted that if White had played 31.Bg7, instead of 31.Bb8, the game would have continued 31...h5 32.a5 a6. One consequence of this is that instead of 31...a5! as played in the game, the move 31...a6 would also have won, although not as quickly. White's problem is that the Queenside Pawns are so weak, that one of them will eventually be lost. This again gives Black an extra Pawn on both sides of the board.

After 33...Bd7 34.Bc7 Kc6 35.Bd8 (35.Bxa5 is again met by 35...b6) 35...b6 36.c4 Kb7 37.Kc3 Bxa4, Fine wrote, 'The rest is not difficult.' It might not be diffiult for a GM, but it is worth studying for anyone else.