How Koga decides
- The move played is a capture. The piece it takes is NOT hanging — it has at least one defender — so this is not a simple free piece.
- Even so, the side capturing comes out ahead once the dust settles: it has more, or more valuable, attackers bearing on the square than the opponent has defenders. The piece is defended, but not defended enough.
- Koga plays out the whole exchange on that one square — each side keeps recapturing with its cheapest available man — and only flags it when the capturing side finishes with a net material gain.
- Attackers and defenders are counted in full, including pieces lined up behind one another: a rook or queen sitting behind another rook, bishop, or queen joins the count as the front piece is traded off.
- A defender hidden behind an enemy slider still counts too — if removing the enemy piece in front of it would reveal a friendly recapture, that guard is included.
- The exchange is read off as, for example, “wins ♞” or “wins ♞♙ for ♝” — the material the capturing side keeps after both sides have taken everything worth taking.
- Not a capture — a quiet move never qualifies; there is no piece being taken.
- Undefended target — if the captured piece had no defender at all, that is a hanging piece, reported on its own page, not here.
- Only an even trade — if playing the exchange out nets nothing (for instance a knight takes a knight guarded by a pawn), there is nothing to win and it is not flagged.
- Given straight back — in a longer line, the gain must still stand a couple of moves later; a capture that is immediately undone does not count.
- A single capture can do two things at once — win the exchange on its square AND, by vacating, uncover a discovered attack elsewhere.
- When both appear as the point of the move, Koga keeps whichever wins more material; on a tie it keeps the favorable exchange, since the capture is what the move actually does.
- But when the discovered attack is the real motif and the favorable exchange only turns up later in the line, the discovered attack suppresses it — the incidental capture is not the headline.
Test positions (the examples that guard against regressions)
Battery pile-up: three attackers beat two defenders
The knight on g5 is defended twice (bishop e3, queen h5) but attacked three times: the knight on e6, plus a bishop-and-queen battery — Be7 with Qd8 behind it on the diagonal. Playing the exchange out on g5, Black ends up a clean knight ahead (wins ♞). The piece was defended, just not enough.
Two attackers against one defender
A constructed position. The knight on d5 is guarded once, by the pawn on e6, but attacked twice — by the bishop on c4 along the diagonal and the rook on d1 down the open d-file. Bxd5 exd5 Rxd5 leaves White a pawn up (wins ♞♙ for ♝): one defender simply cannot hold against two attackers coming from different directions.
Favorable exchange kept over a smaller discovered attack
From a real game (Black to move, 21...Nxd3). The knight on b4 takes the white queen on d3, which is defended only by the e2 pawn — Black wins queen for knight (wins ♕ for ♞). The same move also uncovers the c4 rook's hit on the a4 pawn, but that is small change next to the queen, so Koga keeps the favorable exchange as the headline.
Even trade — nothing to win
A constructed position. The knight on f3 takes the knight on d4, which is guarded by the e5 pawn. After exd4 the count is exactly even — a knight for a knight — so there is no favorable exchange and Koga does not flag it.
Undefended target — that is a hanging piece, not this
A constructed position. The rook on a1 takes the knight on a5, which has no defender at all. A free piece is a hanging piece (its own page); because there was nothing to defend it, this favorable-exchange check does not apply.
A quiet move can never qualify
The opening move 1.e4 captures nothing, so there is no exchange to evaluate and the favorable-exchange check does not fire.
Golden puzzles — every real Lichess puzzle Koga is tested against for this tactic (1)
Lichess 01lp3
Lichess 01lp3 (rating 1479). Solution: Qxg6+ Kh4 Rf4+ Kh3 Qxh5+.