Technical correctness is the foundation on which artistic discussion rests. A beautiful idea cannot rescue a composition that has no intended solution, a destructive cook, an impermissible short solution or an illegal orthodox diagram.
The current WFCC Codex is also crucial because composition has its own conventions for castling, en passant, move rights and the applicability of ordinary FIDE rules.
Soundness is genre- and stipulation-aware. More than one move somewhere in the solution is not automatically a cook.
Legal Diagram
Under orthodox composition convention, a diagram is legal if it can be reached from the initial array by a legal sequence under the applicable rules/conventions, unless the genre/tourney explicitly permits otherwise.
Reachability
Reachability is a retro question: can some legal history connect the initial array to the diagram? It can depend on captures, pawn files, promotions, checks, castling and en-passant rights.
Side to Move
Side to move is part of the stipulation/diagram state. If the conventional starting side does not move first, that must be indicated or deducible under the applicable retro convention.
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.
Check Legality
A diagram cannot simply place a side in an impossible inherited check. Determine whether the checking configuration could legally follow the preceding move under the stipulated rules.
Promotions
Promoted material must be explainable from pawn histories and capture budgets where orthodox legality is required. Several same-type pieces do not by themselves prove illegality because promotions can create them.
Captures
For Captures, ask two questions separately: (1) what do the applicable Codex/rules permit? and (2) does the author's intended solution remain unique/correct under that convention? A soundness claim must state the genre, expected solution count and test method.
Castling Convention
Current WFCC Codex convention: castling is permitted unless it can be proved impermissible. This differs from assuming that historical move rights must always be explicitly demonstrated from a full game record.
En-Passant Convention
Current WFCC Codex convention: an en-passant capture on the first move is permitted only if it can be proved that the immediately preceding move was the relevant double pawn step.
Fairy-Legality Override
A fairy condition can change what legal means. State the condition before applying ordinary reachability, check, capture, castling or mate assumptions.
What Soundness Means
Soundness means that the composition fulfills its stipulation only in the intended manner under the applicable genre conventions and has no disqualifying technical defect.
Unique Intended Solution
The Codex default is that a composition should be solvable only by the author's solution unless special features such as multiple solutions are explicitly stipulated.
Multiple Intended Solutions
Explicit multiple solutions can be integral content, especially in helpmates. More than one solution is therefore not a defect until the stipulation's intended solution count is known.
No Solution
Testing must explicitly search for the intended solution; a program returning none can indicate a genuine no-solution defect, wrong encoding, unsupported fairy rule or software/configuration problem.
Cook
A cook is an unintended alternative solution differing in the first move from the author's solution. It normally destroys soundness unless that multiplicity was explicitly stipulated.
Dual
A dual occurs after the first move when more than one continuation can fulfill the stipulation in the same context. Its seriousness is genre- and phase-dependent; a dual is not the same thing as a second key/cook.
Minor / Major Dual Nuance
The artistic seriousness of a dual depends on where it occurs and what it damages. The Codex explicitly notes that many study duals are tolerated and that judges evaluate seriousness; help-play dualization is treated more strictly.
Short Solution
A short solution fulfills the stipulation in fewer moves than intended. In the Codex, non-helpplay nuance matters: a composition is not necessarily short if at least one full-length line is still required by the stipulation.
Unintended Promotion
For Unintended Promotion, ask two questions separately: (1) what do the applicable Codex/rules permit? and (2) does the author's intended solution remain unique/correct under that convention? A soundness claim must state the genre, expected solution count and test method.
Illegal Variation
For Illegal Variation, ask two questions separately: (1) what do the applicable Codex/rules permit? and (2) does the author's intended solution remain unique/correct under that convention? A soundness claim must state the genre, expected solution count and test method.
Twin Soundness
For Twin Soundness, ask two questions separately: (1) what do the applicable Codex/rules permit? and (2) does the author's intended solution remain unique/correct under that convention? A soundness claim must state the genre, expected solution count and test method.
Computer Testing
Specialist solving software can exhaustively or near-exhaustively test expected solutions, cooks, duals, short solutions and legality under supported conditions. The test is only as trustworthy as the encoded rules, options and version.
Human Thematic Review
Computer correctness does not establish thematic clarity, economy, paradox, originality or good publication explanation. Human specialist review remains a separate layer.
Soundness Practice
For Soundness Practice, ask two questions separately: (1) what do the applicable Codex/rules permit? and (2) does the author's intended solution remain unique/correct under that convention? A soundness claim must state the genre, expected solution count and test method.
Soundness Is a Property of the Whole Task
Technical correctness is broader than finding one intended line. A sound composition must satisfy the stated stipulation under the applicable conventions and must survive all relevant alternatives. The exact defect taxonomy depends on genre, but the diagnostic question is stable: does anything legal happen that changes the intended solution structure?
A useful verification order is:
diagram legality → stipulation encoding → expected solution count → unintended first moves → shorter solutions → duals/dualizations → promotions and special rights → twins/conditions.
This order matters because later artistic analysis is wasted if the task is already broken at a more basic layer.
Intended Multiplicity Is Not a Cook
Helpmates, twins and some fairy forms often publish several solutions deliberately. A second solution is therefore not automatically a defect. The verifier must compare found multiplicity with declared multiplicity. The same principle applies to duals: some are destructive, some minor, some tolerated by genre convention, and some are part of the intended construction. Precision requires classification rather than a blanket “multiple lines are bad” rule.
Legal-Looking Is Not the Same as Reachable
Retro constraints can make a natural-looking diagram illegal even when every piece occupies an ordinary square. Promotions, missing captures, checks, castling rights and en-passant assumptions can encode history. In orthodox forward problems, conventions may deliberately avoid demanding a complete reconstruction of that history; in retro work, history can be the entire task. Always identify which legality model the composition actually uses.