Partial Retrograde Analysis

Retrograde analysis reasons backward from the diagram to determine what legal history could have produced the position. Partial Retrograde Analysis, or PRA, applies when permissions for certain special moves cannot be established independently.

The permissions usually concern the right to castle and the right to capture en passant. Castling is the special move in which a king and rook move together, and it requires, among other conditions, that neither has moved before. In composition it is allowed unless the position proves otherwise. En passant lets a pawn capture as though an opposing pawn, which has just advanced two squares from its starting square and landed beside it, had moved only one square. If en passant is the first move of the solution, that double advance must be provable as the immediately preceding move.

Sometimes the history needed to preserve one right excludes the history needed to preserve another. Rather than choose one version arbitrarily, PRA divides the solution into mutually exclusive parts. Each part assumes one coherent combination of rights, and together the parts must cover every possible combination allowed by the castling and en passant conventions.

When two castling rights depend on each other

If mutually dependent castling rights yield no solution under PRA, the Retro-Strategy convention, abbreviated RS, applies. The first castling move actually played is then deemed permissible. This is a formal way to resolve the dependency, not a claim that both rights coexisted in one history.