Disambiguation — what it means in chess
Adding a file or rank when two like pieces could reach the same square, e.g. Nbd2.
Adding a file or rank when two like pieces could reach the same square, e.g. Nbd2.
[[board startfen="rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1" caption="Two White knights, one destination. Write Nd2 and the notation is ambiguous — the scoresheet must say WHICH knight, and the file letter settles it: Nbd2." pgn="d4 Nf6 Nf3 e6 Nbd2" notes="The starting position. Within five plies, two identical pieces will want the same square — and the notation will have to choose between them. An ordinary queen's pawn opening begins. Black develops. Both White knights are now in play — — and Black waits. Here is the collision: both the b1-knight and the f3-knight can reach d2. Plain Nd2 names two different moves, so the notation adds the origin file: Nbd2, the b-knight. Its partner's version would be Nfd2. One extra letter, zero ambiguity."]]
Disambiguation is algebraic notation's tiebreaker: when two identical pieces can reach the same square, the move gets an extra character naming which one went. The rules are a fixed hierarchy — add the file of origin if that settles it (Nbd2 versus Nfd2, as above); if both pieces stand on the same file, add the rank instead (R1e2 versus R7e2); and in the rare case where neither alone suffices — three queens after promotions, say — write both, as in Qh4e1.
The position above is the everyday case every player meets within their first months of reading chess: knights on b1 and f3, both eyeing d2. Plain Nd2 is not merely sloppy — it is invalid , naming two legal moves at once, and careful databases and arbiters reject it. The file letter costs one character and removes every doubt.
The Endgame Society glossary defines more than 240 chess terms — rules, notation, tactics, strategy, pawn structures, endgames, and openings — in language a beginner can act on rather than dictionary shorthand.