Centurini's rule

Centurini's rule applies to a bishop and pawn versus bishop ending when both bishops move on the same color of squares. The defending bishop tries to stop the pawn from a diagonal the pawn must cross, while the attacking king and bishop try to drive it away.

The first test concerns the defending king. If it can occupy a square in front of the pawn whose color is opposite to the color on which the bishops move, the attacking bishop cannot drive it away directly and the position is normally drawn.

If the defending king is behind the pawn while the attacking king is close, the requirements become stricter. The defending king must attack the pawn and hold the opposition, meaning that the kings face each other so the opponent must give way first. The bishop also needs two useful diagonals with at least two available squares on each. This reserve lets it change diagonals without abandoning the blockade.

If one diagonal is too short, or if its squares are occupied or controlled, the attacker may create zugzwang, a position in which every legal move makes matters worse. The defending bishop can then be forced away from the pawn's promotion route.

The rule is not a formula detached from the board. The pawn's file, meaning its vertical column, king placement, whose turn it is, and genuinely available bishop squares can create exceptions. It identifies the mechanism and guides calculation, but the concrete position still has to be checked.