
A piece is said to attack an opponent's piece if, in the next move, it could capture that piece. A piece is said to defend or to protect a piece of the defender's color if, in case the defended piece were taken by the opponent, the defender could recapture right away. Attacking a piece forces the opponent to respond only if the attacked piece is undefended, or if the attacking piece is of lower value than the attacked one.
The German zwischenzug means "intermediate move"; it is a common tactic that occurs in almost every game: instead of countering a direct threat, which the opponent expects, a move is played which poses an even more devastating threat, often an attack against the queen or the king. The opponent has to counter that threat first, and this will ideally change the situation to his disadvantage.
When you plan your tactics, you should always watch out for a zwischenzug. Don't assume that the opponent has to counter your threats immediately. It is good practice to always check whether your opponent has a check or a move threatening your queen. Conversely, anticipate your opponent's threats and plan a surprising zwischenzug.
Often it is necessary to throw the opponent's position out of balance by first sacrificing some material, to be regained with interest a couple of moves later. Pawn sacrifices in the opening are known as gambits; they are usually not intended for material short-term gain but instead to achieve a more active position.
Direct attacks against the enemy king are often started by sacrifices; a common example is a bishop sacrificing itself on h7, checking the king on g8 who has to take the bishop, after which the white queen and knight develop a fulminant attack.
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