An honest reconstruction.

What chooses the move?

The site’s own JavaScript chooses the move. The locally bundled chess.js library handles legal moves and game state; it supplies no chess AI. No Stockfish, neural network, opening book or endgame database is involved.

Material and position are separate

Material uses pawn 1, knight 3, bishop 3½, rook 5 and queen 10. The king is handled through checkmate rather than a tradable piece value.

Each root candidate receives a searched material ratio (own total divided by the opponent’s) from the calculating side’s perspective. The best material score wins; tied candidates are ranked by their immediate position-play score. Exact ties use a deterministic alphabetical ordering of move notation. The ratio and positional tie-break follow the typescript; alphabetical final ties are a browser convention. A ratio of 1 means equal material. Draws also score 1. If only the opponent has no non-king material, a finite value of 1,000 replaces the undefined ratio; checkmate always takes priority.

The position-play calculation

Mobility rewards the square root of available moves, counting a capture twice. Minor pieces and rooks receive defence credit. Pawns receive advancement and non-pawn-defender credit. The king receives mobility and castling terms, offset by exposure estimated through a hypothetical friendly queen on its square. Checks and mate threats add small bonuses.

The detail panel reports these terms for the engine’s side immediately after its candidate move. It does not subtract the opponent’s complete positional score. It reports the total and the change from the position before the candidate move. The latter makes it easier to compare with the historical hand calculations.

Search choices in this version

• Examine every legal root move and every legal reply: two plies.
• Beyond that, extend recaptures on the previous capture square, favourable-value captures, captures of undefended pieces (no opposing geometric attack on the capture square after the capture), promotions and immediate mating moves.
• When in check, examine legal evasions. At extension nodes, include the current material value only when at least one legal move is not considerable. If all moves are considerable, follow them without a standstill option.
• Use alpha-beta bounds to avoid material-search work that cannot change the result.
• Limit extensions to six additional plies, evaluated at the boundary, and approximately 6,000 visited nodes per root candidate. The limit is checked at node entry; remaining sibling visits may take the total slightly above it.

Limits apply independently to each candidate. A marked score means a boundary was reached, not that a quiet position was proved. A forced sequence beyond that boundary can be missed. This bounded search is a deliberate browser adaptation of the historical idea of following significant continuations.

Conventions that affect the answer

• Square roots are rounded to one decimal place, following the hand calculation described on page 7 of the typescript. Positional totals are kept to tenths so floating-point noise does not decide an apparent tie.
• Mobility uses legal moves for a hypothetical extra turn, with any generated capture of the opposing king excluded. Defence uses geometric attacks, including pinned defenders.
• King exposure uses open queen rays and doubles captures. It does not replace the king in the rules library and re-test king legality.
• A future castling bonus requires at least one retained castling right; an immediate bonus requires a legal castle. The performed-castling bonus applies only to the root castling move. In this implementation a currently legal castle scores two points before it is played (rights plus availability), and the completed castle scores one (performance). The wording can also be read cumulatively: that alternative remains a documented ambiguity, not a resolved historical fact.
• A mate threat means the calculating side would have a legal mate in one if allowed another move. This hypothetical test is skipped when the opponent is already in check.
• Promotions and check evasions are followed as practical tactical extensions; these are explicit reconstruction conventions.
• Threefold repetition and the 50-move rule are automatically treated as draws in this interface rather than requiring a claim. Insufficient material uses chess.js detection, not an exhaustive solver for every possible dead position.

A worked opening comparison

Opening the e-file gives both moves the same piece-mobility gain. Advancing to e4 earns 0.2 more pawn credit, but exposes an extra square in front of the king. With rounded square roots, that exposure costs another 0.4. The net result favours e3 by 0.2.

Other components are unchanged. Turing’s typescript records e4 with +4.2. Matching that arithmetic does not establish that every later move or search decision has been reconstructed exactly.

*As reproduced in Friedel’s ChessBase reconstruction article. These are changes in the immediate positional score, evaluated along the recorded game, not the engine’s preferred moves or its searched material results. Differences between the surviving versions are shown explicitly.

What the scores do not mean

A positional score of +15 is not a fifteen-pawn advantage. It is a sum of this procedure’s mobility, defence and other terms. Material and positional scores are separate columns, both oriented towards the calculating side. They are not win probabilities.

What remains to validate historically

The September 2026 review checked the typescript’s material ratio, considerable-move rules and positional terms, and corrected the ratio and the all-considerable-moves case. The October pass adopted hand-calculation rounding, excluded hypothetical king captures from mobility, and checked five opening-score changes. Castling-bonus interpretation, the mate-threat convention and wider position-by-position comparisons remain open. Replaying the Glennie score is already possible, but the engine’s disagreements are not automatically historical errors or evidence of mistakes by Turing.