How Koga decides
- One of our long-range pieces — a rook, bishop, or queen — attacks a valuable enemy piece along a rank, file, or diagonal.
- Directly behind that piece, on the same line, sits a second enemy piece of equal or lesser value.
- When the front piece steps out of the way, the piece behind it is exposed and falls. It is the reverse of a pin: here the more valuable piece is in front.
- The front piece must be at least a minor piece — a lone pawn out in front is never worth calling a skewer.
- Normally the front piece must be worth more than our attacker, so that forcing it to move is a real gain.
- Exception: if the front piece is undefended, our attacker simply wins it for free, so we still flag the skewer even when the values are equal (for example a queen attacking an undefended rook with a knight behind it).
- The piece behind must be worth no more than the front piece — otherwise the alignment is a pin, not a skewer.
- If the front piece could answer by simply capturing our skewering piece, and that skewering piece is undefended, the skewer is not forcing and we do not flag it.
- New skewer (motif) — the move creates the alignment.
- Pre-existing skewer (enabling) — the alignment was already on the board, so it cannot be the reason the move is best and does not suppress other motifs.
- Found deeper in the line (follow-up) — the alignment only appears a few moves into the engine's main line.
Test positions (the examples that guard against regressions)
Skewer on the a-file
A constructed position. Our rook on a1 attacks the black king on a5 along the a-file, with the black rook on a8 directly behind it. The king must move out of check, and the rook on a8 is then ours.
Golden puzzles — every real Lichess puzzle Koga is tested against for this tactic (11)
Lichess 007tv
Lichess 007tv (rating 1116). Solution: Qxf7+ Kd8 Qf8+ Kd7 Qxa8.
Koga flags skewer as a follow-up here — it fires later in the line, on Qf8+, not on the move shown above.
Lichess 02meJ
Lichess 02meJ (rating 1366). Solution: Bh3+ Kxf3 Bxd7 cxd7 Ke7 d8=R Kxd8.
Lichess 0039T
Lichess 0039T (rating 1072). Solution: Rb1+ Kf2 Rxh1 Rxa7+ Kf8.
Lichess 02a3l
Lichess 02a3l (rating 1117). Solution: Rb3+ Kc5 Rxb8.
Lichess 00TiV
Lichess 00TiV (rating 1140). Solution: Ra3+ Kc2 Rxf3.
Lichess 02CX1
Lichess 02CX1 (rating 1475). Solution: Rxa2+ Kb5 Ra5+ Kc6 Rxh5.
Koga flags skewer as a follow-up here — it fires later in the line, on Ra5+, not on the move shown above.
Lichess 022ky
Lichess 022ky (rating 1295). Solution: Qe3+ Kd6 Qxe8.
Lichess 02xCt
Lichess 02xCt (rating 1577). Solution: Ra1+ Kd2 Rd1+ Ke3 Rxd5.
Koga flags skewer as a follow-up here — it fires later in the line, on Rd1+, not on the move shown above.
Lichess 001m3
Lichess 001m3 (rating 1459). Solution: Rh1+ Ke2 Rxa1.
Lichess 00baa
Lichess 00baa (rating 2076). Solution: Be5 Nxf6+ Qxf6 Qc4 Bxb2.
Lichess 06s4Z
Lichess 06s4Z (rating 1973). Solution: Rxe4 Qxe4 Re8.
Koga flags skewer as a follow-up here — it fires later in the line, on Re8, not on the move shown above.