Nbd2, R1e2: how to tell two pieces apart

Two knights, one square. White’s knights stand on b1 and f3, and both of them can go to d2. Plain Nd2 would leave the reader guessing, so the move takes one extra character: Nbd2 for the knight from b1, Nfd2 for the one from f3.

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The rule (Appendix C.9) is a short ladder and you stop at the first rung that works. Use the file the piece came from. If both pieces stand on the same file, use the rank instead. If a file alone and a rank alone both leave it ambiguous, write both.

Same file, so use the rank

Rooks doubled on a file are the everyday case. With white rooks on e1 and e5 and both able to reach e2, the file letter helps nobody: they are both on the e-file. The rank does the work instead, and you write R1e2 or R5e2.

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Rooks on a1 and h1 going to d1 are the other common case, and there the files differ, so it is Rad1 and Rhd1. When the disambiguated piece captures, the extra character comes before the x: Nbxd2, R1xe2, Rhxd1, and a check still goes on the end, R1xe2+.

Both a letter and a number are needed only when three pieces of the same kind can reach one square, which in practice means somebody has promoted. Queens on d1, h1 and h5 all bear down on d5: the one on h1 shares its file with h5 and its rank with d1, so neither half is enough on its own and the move is written Qh1d5. Expect that about once every thousand games.

Only legal moves count

Here is the rung nobody mentions. Ambiguity is decided by what is legal, not by what looks plausible on the squares.

If one of those two knights is pinned against its own king and cannot legally go to d2, then only one knight can make the move, and plain Nd2 is correct. Write the extra letter anyway and nothing bad happens; the move is still readable and a strict program will tidy it up. But that is why a game score sometimes shows Nd2 in one place and Nbd2 in another with the same two knights on the board.

Two footnotes. Pawn captures always carry their file, exd5, whether or not a second pawn could take there, which is the pawn rule rather than this one. And bishops almost never need disambiguating at all, since your two bishops live on opposite colours and can never reach the same square; the exception is a second bishop that arrived by promotion, which is a curiosity rather than a problem.

Knights are the pieces this comes up with most, mainly because they approach a square from two directions at once. If their geometry still feels awkward, how the knight moves is the page to read, and the fork puzzles are where two knights aiming at the same square usually turns out to matter.