Opposite-Colored Bishop Endgames: Building the Fortress

Watch the method: press play to see the winning idea run, or step through it move by move.

Opposite-colored bishop endings are the great heartbreaker of practical chess: the most material-defying draws on the board, and the reason 'two pawns up' sometimes means nothing. One number matters more than the pawn count: the color of the squares in front of the pawns.

The principle. Each bishop rules half the board and is a tourist on the other half. If the defender can place his king and bishop on the squares the pawns must cross, the attacker's extra bishop contributes exactly nothing to the fight: it moves on the wrong color forever. The attack is not two pieces against one; it is one piece against two.

The division of labor is precise. Connected pawns advance on alternating colors, so the defense splits the job: the king plants itself on the advance square of one color, the bishop patrols the diagonal covering the other. Get the assignment backwards and zugzwang unravels everything; get it right and the fortress needs no further moves of substance.

The boundary: connected passers that have both reached the sixth rank win, blockade or not, with one exception: a rook's pawn beside a knight's pawn (a6 and b6, or g6 and h6), where the defending bishop just waits on the long diagonal into the corner and holds the draw. On the fifth or earlier, the fortress holds with exact play. This drill starts at the critical moment where one bishop move decides which side of that boundary you live on. The attacker never gives up a half point: he will spend fifty moves probing the wall; your job is to make the wall boring.