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Chess Tactics

The essense of chess tactics is to make a single move which threatens the opponent twice - unless they have some counter tactic to either deal with both threats, they will end up in a worse position, because you will be able to follow through on one of your threats with no trouble.

The simplest tactic is the fork - where one piece directly attacks two or more pieces. This is normally done by knights, because of their unique movement, but any piece may fork - including the king!

Other tactics include the following:

• Pin - A bishop, rook, or queen gets in line with two of the opponent's pieces, where the intermediary piece is less valuable than the one it is blocking. If the opponent moves the intermediary piece, the farther one can be captured. If the far piece is a King, it's called an absolute pin, because the intermediary piece cannot be moved.
• Skewer - Similar to a pin, but the intermediary piece is the more valuable one, and the opponent usually ends up having to move it, and loses the far piece.
• Discovered attack - By moving one piece out of the way, exposing a piece behind it so that it can attack another. The attacking piece can be a bishop, rook, or queen - and if the moved piece also makes an attack, the defender has a difficult choice as to which threat to defend against.
• Double check - This is a type of discovered attack where both attacking pieces check the opponent's king, which forces the king to move, as it is impossible to block both checks in a single move.
• Undermining -By capturing a piece which is defending another, the opponent has to chose between recapturing the attacking piece, or defending their newly undefended piece.
• Overloading - By gicing a defending piece too many squares that it has to defend, it has to give up defending some of them. This can result in hung pieces and other bad situations for the defender.
• Interference - By moving a piece in between two long-range pieces that are defending eachother, the opponent is forced to either defend against the new threat that the interfering piece makes, or to defend the pieces that are no longer defending eachother.
• X-Ray Attack - A piece defends another through an opponent's piece - if the opponent captures one of them, the other can recapture. http://en.wikipedia.org/wiki/X-ray_(chess)
• Zwischenzug - A counter-tactic where a player makes an intermediary move which poses a more dangerous threat than their opponent's most recent attack. This is a way to get out of situations caused by tactics used against you. Usually the intermediary move has to create a check on the opponent's king, since they are then compulsed to defend against it.
• Zugzwang - A situation that comes up usually in endgames, where the opponent would rather pass their turn than move. Of course, you can't pass your turn in chess, so they are compulsed to move to a position worse than the one they are in.

Tactics like these can be strung together to form combinations. An example of this is known as a windmill - a series of checks and discovered checks where the attacker can pick off defending pieces on every other move, and the place their pieces in a better position at the end. This usually happens where the opponent's king becomes stuck in a place where it has only a couple squares to move to. A good example is in this game, starting on move 25 with a queen sacrifice to set up the windmill.