Retroanalysis reasons backward from the diagram. Instead of asking only what can happen next, it asks what must, may or cannot have happened before.
Proof games turn that logic into a construction task:
initial array → exact diagram in the stipulated number of moves
The central resources are capture accounting, pawn history, promotion history, move rights, tempo and route constraints.
What Retroanalysis Is
Retroanalysis deduces prior play from the present diagram using legality constraints. It can establish last moves, captures, promotions, move rights and even impossibility.
Last Move
The possible last move can determine legality, check status and en-passant rights. Retroanalysis should distinguish a last move exists from this unique last move is forced.
Legal History
A legal history is at least one move sequence from the initial array to the diagram under the applicable rules. Retro problems may ask for existence, uniqueness or consequences of such histories.
Capture Accounting
Capture accounting uses missing material and pawn-file changes to constrain which units must have been captured and where captures could have occurred.
Pawn History
Pawns encode history because they move only forward and change files only by captures. Their current file/rank can sharply constrain prior captures and promotions.
Promotion History
Extra material may imply promotion, while pawn/capture accounting constrains which pawn could have promoted and whether the resulting piece identity is possible.
Piece Identity
Two identical-looking rooks/queens/bishops may have different histories: original or promoted, moved or unmoved. Retro conclusions can depend on identity rather than piece type alone.
Castling Rights
Castling rights are historical properties: king/rook movement history matters. Apply the Codex castling convention and any retro proof required by the stipulation.
En-Passant Rights
En-passant rights depend on the immediately preceding double pawn move. Under the Codex first-move convention, that last move must be provable.
Retro Check Legality
Retro check legality asks whether the current checking configuration can have arisen from a legal immediately preceding move. A side cannot simply be left in an impossible inherited double check or with both kings illegally exposed; reconstruct candidate last moves and eliminate those that violate movement, capture or king-safety constraints.
Proof Game
A proof game asks for a legal sequence from the initial position to the diagram in the exact or shortest stipulated number of moves. The composition is the unique/precise route, not merely proof that the diagram is reachable.
Initial Position → Diagram
Proof-game solving works backward and forward simultaneously: every tempo, capture, route and pawn move must fit both the starting array and the final diagram.
Exact / Shortest Length
A shortest proof game stipulates minimal length; an exact proof game requires arrival in the stated length. Extra waiting moves can invalidate uniqueness or minimality.
Tempo
Tempo in retroanalysis is a historical resource. A proof game may require a piece to spend exactly one or more otherwise unnecessary moves so that both sides arrive at the diagram on the stipulated move number; triangulation-like routes, reversible piece moves and pawn irreversibility determine whether the tempo budget is feasible.
Route
Route analysis constrains how a piece could travel from its initial square to the diagram. Blockers, pawn moves, captures and the exact number of available tempi can make one route forced; a geometrically short route may be historically impossible because the necessary squares were occupied at the relevant time.
Capture
Every retro capture leaves material evidence. Missing pieces, pawn file changes and promoted material impose a capture budget that must be consistent across both armies; assigning one missing unit to the wrong capture can make an otherwise plausible history impossible.
Promotion
Promotion history explains extra queens, rooks, bishops or knights and can also account for missing pawns. A valid reconstruction must identify a pawn route to the promotion square, the captures needed to change files and enough tempi to promote before the diagram is reached.
Uniqueness
Uniqueness in a retro or proof game means that the relevant history, last move or exact route is forced under the stipulation—not merely that one legal reconstruction exists. Alternative histories must be excluded by capture accounting, move rights, tempi or piece identity before a unique solution can be claimed.
Solve a Proof Game
To solve a proof game, inventory the final diagram against the initial array, count missing material and pawn-file changes, identify pieces that must have moved, and compare the total tempo budget with the stipulated length. Build routes for both sides in parallel, because one side's clearance move can be the other side's only path or capture opportunity.
Retractor
A retractor requires one or more moves to be taken back under retro rules before a forward objective can be achieved. Specify who retracts, alternation and any fairy/retro convention.
A-to-B
An A-to-B construction asks for a legal or otherwise stipulated sequence from one position to another. It is a constructional/retro task whose length and rule set must be explicit.
Construction Task
A retro construction task asks the composer to create a diagram that forces a desired historical conclusion—unique last move, castling right, promotion identity, proof-game route or similar constraint. The construction must remain legal and should express the retro idea economically rather than hiding it behind unnecessary material.
Fairy Retro
Fairy retroanalysis combines backward reasoning with a non-orthodox rule set. The fairy condition can change captures, rebirth, ownership, movement or check, so ordinary retro assumptions must be replaced before reconstructing history; the condition and its exact version are part of the proof.
Constraint-Satisfaction Method
Retroanalysis is naturally a constraint-satisfaction problem. Each fact—missing material, pawn file, check, move right, square occupancy, tempo—reduces the set of possible histories; solving proceeds by intersecting those constraints until a history is found, several remain, or contradiction proves illegality.
Retro Mastery Set
Retro mastery should mix last-move deduction, capture accounting, castling/en-passant rights, promotion identity and an exact proof game. The solver must distinguish possible, forced and impossible historical claims and provide the constraint that rules out the principal alternative.
Retroanalysis Is Constraint Solving
Forward chess asks what can happen next. Retroanalysis asks what must, may or cannot have happened before the diagram. The strongest method is bookkeeping: account for missing material, pawn capture files, promotions, checks, impossible identities and special rights, then intersect those constraints until candidate histories disappear or become forced.
Do not treat every retro as a narrative reconstruction. Some tasks ask for the last move, some for legality, some for exact history, and proof games ask for a legal route from the initial position with a stipulated length. The required degree of reconstruction comes from the stipulation.
Proof Games Combine Geometry and Tempo Accounting
In a proof game, a piece can reach the right square by many geometric routes but only some fit the move budget. Count tempos, captures, unblocking moves and promotions. A single apparently harmless detour can make the entire history too long. Conversely, a move that looks strategically absurd may be forced because it performs a necessary tempo function.
Special Rights Are Historical Evidence
Castling and en-passant rights connect the present diagram to prior moves. In retro problems they can become deductions rather than administrative metadata. Treat them as evidence: what must be true for the right to exist, and what history would destroy it?