Vacating sacrifice
A vacating sacrifice gives up a piece so that it leaves, or is captured on, a square or line that another of your pieces needs, usually to deliver mate or win material.
Sometimes your best piece is stuck behind one of your own. The queen's diagonal is blocked by a knight, or the knight itself stands on the only square the queen could use. A vacating sacrifice solves this by throwing the blocking piece away, so the way opens for the piece behind it.
To spot the idea, ask which of your pieces is aimed at the enemy king but blocked by a friend. Then ask whether that friend can move with a check or a capture, so that your opponent has no time to react. If the blocker lands on a square where it must be taken, or gives a check that forces a reply, the line it was blocking opens with tempo.
It works best when the sacrificed piece comes with a check or a threat. Without one, the defender can simply ignore it and cover the newly opened line. Also check that the capturing piece does not itself land on a useful square, since that can ruin the follow-up.
To defend, watch for your own pieces that are lined up behind a blocker, and keep the key square guarded twice. A defender who sees the vacated line early can often move the king away from it.
In the diagram White plays Nh6+. The knight on f5 stands in front of the queen on d3, which is aimed at h7. After Nh6+ gxh6, the diagonal is open, and Qxh7 is mate because the knight on g5 guards the queen.
A neighboring idea is Clearance sacrifice. A clearance sacrifice gives up material to remove a piece from a square, file, rank or diagonal, so that another of your pieces can use that line or square for a winning or mating move.
Compare it with Windmill. A windmill is a repeating pattern in which a rook moves away to give a discovered check, captures a piece, returns with check, and repeats, winning another enemy piece with each swing.
Also worth reading is Battery. A battery is two of your pieces lined up on one file, rank or diagonal, with the rear piece backing up the front one so that both press on the same target.
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Related terms
In a discovered attack one of your pieces steps off a line, and that move unmasks a bishop, rook or queen standing behind it, so a single turn produces two threats.
A clearance sacrifice gives up material to remove a piece from a square, file, rank or diagonal, so that another of your pieces can use that line or square for a winning or mating move.
A combination is a forcing sequence of moves, usually built from sacrifices and checks, in which every move leaves the opponent few or no choices and the sequence ends in a concrete gain such as mate or material.
Deflection is the tactic of dragging an enemy defender off its post, often by a threat or an offered capture, so that what it was holding together falls apart.
A battery is two of your pieces lined up on one file, rank or diagonal, with the rear piece backing up the front one so that both press on the same target.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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