How Koga decides
- An x-ray is when one of our long-range pieces — a rook, bishop, or queen — acts through an enemy piece standing on its line, reaching the square behind it as if that enemy piece were transparent.
- Koga looks for this idea inside the engine's main line, not just in the starting position, because the alignment usually appears in the middle of a forcing sequence.
- Exchange — three captures in a row land on the same square, and the last capturer was sitting behind the second one, so it recaptures through the spot the middle piece just left.
- Support mate — after such an exchange a slider behind the contested square delivers (or completes) mate, adding a line of attack the front piece could not, either because we sacrificed down to it or because it attacks along a different kind of line.
- Defense — we land a piece on a square the enemy attacks, and a slider of ours defends that square through the very enemy piece attacking it: if the enemy captures, our slider recaptures.
- Sandwich — we move a slider so an undefended enemy piece is trapped on one line between it and a second slider of the same type behind.
- Off-ray refutation — if the piece that finishes the exchange does not stand on our slider's line, then it, not our slider, won the square. The alignment was only a coincidence and we do not report it.
- Delayed refutation — the same applies when a check is thrown in first: we follow the forcing check-and-reply chain, and if the enemy recaptures the contested square anywhere along it, our slider was still lost.
- Losing defence — for an x-ray defence we require the slider to be genuinely needed (without it the square has too few defenders) AND that the worst-case exchange does not still win material for the enemy. If the defended piece is worth far more than the attacker, the defence is only a phantom and is rejected.
- When the main line ends exactly on the third capture of an exchange, the engine never showed a refutation. We take that silence as a sign the alignment holds and report the x-ray.
Test positions (the examples that guard against regressions)
Exchange x-ray at the end of the line
A constructed position. The line runs Bxf5, Qxf5, Rxf5: the rook on f1 was sitting behind the black queen on f3, so when it recaptures on f5 it acts through the square the queen vacated — a textbook exchange x-ray. The line stops right on that recapture, so with no refutation shown the alignment is trusted.
Off-ray piece finishes the exchange (rejected)
From a real game. The captures pile up on f6 — Bxf6, exf6, Qxf6 — and the queen on b2, the f6 square, and the bishop line up on the long diagonal. But the piece that recaptures last on f6 is the g5 pawn, which sits off that diagonal. Because an off-ray piece won the square, the alignment is only geometric and no x-ray is reported.
X-ray defence that loses material (rejected)
The same real-game position, now as a single move Bxf6. One might read the queen on b2 as defending f6 through the pawn on e5, but the f6 square is also hit by the g5 pawn off that diagonal, and the worst-case exchange still loses material. The defence is a phantom, so no x-ray is reported.
Defended piece too valuable (rejected)
A constructed position. The queen lands on e5, attacked by the pawn on d6, with the bishop on c7 x-raying through the pawn to e5. But dxe5 wins the queen and Bxe5 wins back only the pawn — a heavy net loss. Because the exchange still wins material for the enemy, the x-ray defence is rejected.
Refutation delayed by a check (rejected)
A constructed position. The exchange Bxf5, Qxf5, Rxf5 forms the same x-ray shape as the accepted case, but Black inserts the in-between check Nh3+ forcing Kh1, and only then does the bishop on c8 — off the rook's line — capture on f5. Following the forcing check chain shows the rook is still lost, so the x-ray is rejected.
Golden puzzles — every real Lichess puzzle Koga is tested against for this tactic (10)
Lichess 0AQRf
Lichess 0AQRf (rating 1622). Solution: Qxg2+ Rxg2 Rxg2#.
Lichess 02ouU
Lichess 02ouU (rating 2175). Solution: Qa8+ Kd7 Bxb5+ c6 Bxc6+ bxc6 Qxc6#.
Koga flags x-ray as a follow-up here — it fires later in the line, on Bxc6+, not on the move shown above.
Lichess 0HTHp
Lichess 0HTHp (rating 1878). Solution: Qxc8+ Qxc8 Rxc8+.
Lichess 0G4Pc
Lichess 0G4Pc (rating 1268). Solution: Rh1+ Nh7 Rxh7+ Qxh7 Qxh7#.
Koga flags x-ray as a follow-up here — it fires later in the line, on Rxh7+, not on the move shown above.
Lichess 0H0iH
Lichess 0H0iH (rating 1919). Solution: Nh2+ Qxh2 Qxh2 Re8+ Rxe8.
Lichess 0KR02
Lichess 0KR02 (rating 1932). Solution: Rd1 Qc7 Rd8+ Qxd8 Qxd8+.
Koga flags x-ray as a follow-up here — it fires later in the line, on Rd8+, not on the move shown above.
Lichess 0KGFf
Lichess 0KGFf (rating 1542). Solution: Qxc8+ Rxc8 Rxc8+.
Lichess 0Kdxm
Lichess 0Kdxm (rating 1494). Solution: Bc4+ Kh8 Rxf8+ Rxf8 Rxf8#.
Koga flags x-ray as a follow-up here — it fires later in the line, on Rxf8+, not on the move shown above.
Lichess 069H6
Lichess 069H6 (rating 923). Solution: Rc8 Rxd7 Rxd8+ Rxd8 Rxd8+.
Koga flags x-ray as a follow-up here — it fires later in the line, on Rxd8+, not on the move shown above.
Lichess 0HAvW
Lichess 0HAvW (rating 1645). Solution: Bxg7+ Rxg7 Rxg7 Qxd3+ Ka1.