Interference (chess)
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Interference is a chess tactic which seldom arises, and is therefore often overlooked. Interference occurs when the line between an attacked piece and its defender is interrupted by sacrificially interposing a piece. Opportunities for interference are rare because the defended object must be more valuable than the sacrificed piece, and the interposition must itself represent a threat.
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(The following moves are given in algebraic chess notation.) In the position at left, White to play will apparently be obliged to retreat the knight from f5, because the squares to which it could advance are all guarded. However, the interference move 1.Nd6+ interrupts the black rook's defense of the black queen. If Black plays either 1...cxd6 or 1...Bxd6, White will obtain an overwhelming position with 2.Qxd2. Therefore Black has no better play than 1...Rxd6 2.exd6 Qxe2 3. Rxe2 Bxd6, conceding a small material advantage to White.
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A more subtle example of interference occurs when the interposing piece interrupts two lines simultaneously. In this case, the moving piece does not have to pose a threat by itself. Instead, it makes the opponent trip over his own feet, so to speak, because capturing the offending piece will necessarily break one line of defense or the other.
In the position at right, White is at a material disadvantage, and apparently can't queen the a-pawn because the black bishop guards the queening square. However, 1.Nd5 interferes with the bishop and with the black rooks' defense of each other. If 1...Bxd5, 2.Rxd8 is crushing. The best Black can do is 1...R2xd5, interfering with the bishop's guard of a8 and allowing 2.a8Q.
Although interferences are quite rare in actual play, they are a common theme in chess problems. The device in the last example above, in which a sacrifice occurs on the intersection of the defensive lines of two differently moving pieces, is known to problemists as a Novotny. Various other types of interference are given specific names in problem terminology, including the Grimshaw, Plachutta, Anti-Bristol, Holzhausen and Wurzburg-Plachutta.