How Koga decides
- The move we judge must be a capture of an enemy piece or pawn.
- If the captured square has no enemy defender, we take the piece for free — there is no recapture to give anything back. That is a hanging piece.
- It applies to anything from a pawn to the queen: the only test is that nothing was guarding it.
- A piece is defended if any friendly man — including a pawn — attacks the square it stands on. A pawn guard is enough to make a capture an even trade rather than a free win.
- We also honour x-ray defenders: if a friendly piece would guard the square once an enemy slider in front of it is removed, the piece is treated as defended.
- If the piece is defended but the capture still wins material after the full exchange, that is the separate Insufficiently Defended pattern, not a hanging piece.
- When the engine line runs several moves on, we re-check the material balance a couple of moves later.
- If the side that grabbed the piece has handed the advantage straight back, we do not call it a hanging piece — the point is a lasting win, not a momentary one.
- A hanging piece stands on its own: when the move is also a Fork or a Skewer, all of those ideas are reported together — neither pushes the hanging piece aside.
- A hanging piece in turn outranks a bare Deflection label.
Test positions (the examples that guard against regressions)
A free knight
A constructed position. The rook on a1 takes the black knight on a5, which has no defender. Nothing recaptures, so White simply wins the knight.
Even a pawn counts
A constructed position. The d4 pawn captures the undefended e5 pawn. There is no guard on e5, so the pawn is won outright — the pattern is not reserved for pieces.
Defended — not hanging
A constructed position. Nxe5 grabs the e5 pawn, but that pawn is guarded by the d7 knight and the f6 knight. Because a recapture is waiting, this is not a free win and the pattern does not fire.
A pawn guard is enough
A constructed position. Black's Nxd5 takes the white knight, but it is defended by the e4 pawn. A single pawn guard turns the capture into an even trade, so it is not a hanging piece.
No capture, no hanging piece
A constructed position. The opening move 1.e4 takes nothing, so there is no piece to win — the pattern only ever applies to captures.
Knight snaps off a loose bishop
Lichess 002bK. After the bishop check ...Bb5+, the knight replies Nxb5, winning the bishop, which had no defender on b5.
Winning the queen for free
Lichess 00QQS. After Black plays ...Qxd4, the bishop answers Bxd4, capturing the black queen, which sat undefended on d4.
Recapturing a knight that has no guard
Lichess 0095W. After Nxe5 by White, Black answers fxe5, winning the knight on e5 outright — White had nothing defending it.
A later skewer must not steal the credit
Bxg2 wins the undefended bishop on g2 — that free piece is the real point of the move. Several moves later the bishop happens to line up against a knight with a pawn behind it (a follow-up skewer that the line never cashes in). Because that skewer only appears later, it does not push aside the hanging piece, which stays the move's own idea.