Centurini Position: Bishop and Pawn vs Same-Colored Bishop

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

Same-colored bishop endings with one pawn are decided by a rule the Italian master Luigi Centurini worked out in the 1850s and tablebases confirmed to the letter: everything depends on the two diagonals that pass through the queening square, and on whether the defending bishop can actually use both.

The two-diagonals rule. A defending bishop guarding the promotion square from a LONG diagonal can be challenged: the attacker's bishop occupies the same line, and the defender must either trade into a lost pawn ending or retreat to the OTHER diagonal through the queening square. If that second diagonal is four squares or longer, the defender keeps switching between the two and, with his king holding the rear opposition, the game is drawn. If it is shorter than four squares and your king and bishop together can cover the whole of it, the deflection wins.

Why a knight-pawn is the dream. For a pawn on b6, the second diagonal through b8 is a single square, a7, and the pawn itself controls it. The defending bishop effectively has one diagonal, which by the rule is one too few. The defender's real hope is his king sitting on b8, so the attacking king must first walk around and lever it out, using zugzwang delivered by bishop tempi.

The move-order trap in this drill is instructive: promoting-minded pawn pushes throw the win away instantly, while the two quiet king moves both win. Same-colored bishop endings are king-and-geometry endings; the pawn moves last.