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.
- Usually the line wins the piece behind. Sometimes the front piece takes our skewering piece instead and is recaptured (a queen for a rook).
- We still call that a skewer, but only when the front piece had a safe square off the line, and every such square lets our piece win the piece behind for at least what the front piece gave up.
- A front piece with no safe square is trapped, not skewered; a front piece that can step away cheaply is not skewered either.
- 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.
Queen gives itself up for the rook
lichess.org/dzgVQfgv: after 32...Qc2?? White plays 33.Rc1. The queen is attacked with the c7 rook behind it. Every safe queen move lets Rxc7 win the rook, so Black gives the queen for the rook instead (Qxc1 Rxc1).
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.