When one of your pieces is placed so that it attacks a square occupied by another of your pieces, the former is said to be guarding the latter. When your opponent captures the guarded piece, you can recapture with the guarding piece. Note that if you have a piece that is pinned to your king by an opposing piece (see below for a description of pins), it can only guard other pieces against capturing by the enemy king, but not from other enemy pieces, since it is unable to move or capture.
Batteries are formed when two or more pieces work together. The most common kind of battery is the doubling of rooks on a file. Other batteries can be formed by rook-queen or bishop-queen.
A triple battery can be constructed with the queen and both rooks. This is known as Alekhine's gun (diagram at right) and can be extremely dangerous. It is often advantageous to place the queen behind one or both rooks as it is a more valuable piece.
The Exchange 
In almost any game, a player will have the opportunity to take one of his opponent's pieces in exchange for one of his own pieces. This should however NOT be done for its own sake! Initiate an exchange only when it benefits you. Benefits can include, but are not limited to:
- Material advantage - Queen for a minor piece, queen for rook, rook for a minor piece, a piece for a pawn or two, etc.
- Doubling pawns - Take when taking back means the doubling (or tripling) of your opponent's pawns on the same file.
- Opening up the king's defenses - Take when taking back means moving a pawn that exposes the king.
- Removing a defender - Take when the piece being taken is providing an essential service for the opponent.
- Blunting an attack - When you are being attacked, often a well timed exchange will leave your opponent with too few pieces to keep up the attack.
- Gaining space - In a cramped position, having more pieces can actually be a disadvantage because the pieces get in the way of each other. If your opponent has a space advantage, exchanging pieces can lessen the advantage and make the resultant less confining.
- Improving a material advantage - if you are ahead material, exchanging pieces will usually benefit you (note - pieces, NOT pawns). Similarly, if you have an extra pawn, trade pieces that may otherwise be used as a sacrifice to prevent pawn promotion.
Forks and Double Attacks 
Sometimes a piece can be in position to attack two enemy pieces at once. This is called a fork or a double attack. All pieces can fork, even pawns, but knights have a reputation for making especially vicious forks because they can jump over other pieces (and can attack another piece from a square that the enemy piece cannot attack, unless the enemy piece is another knight.)
Forking with check 
A knight fork that attacks the king from an unguarded square is very powerful. The opponent must then move his king to safety and the other piece in the fork has no chance of escape.
The Royal Fork 
A royal fork is one involving both your opponent's king and queen. In the example shown here, white's knight on f7 has engaged black in a royal fork. Black will be down the exchange of a queen for a knight.
Forks do not always win material 
If every time you saw a fork you played it, you would be making a mistake. You have to examine the different possibilities to escape the fork. In the queen fork on the right white has just played 1.Qd4+ forking the black king and rook. However black can play 1. ...Rg7 blocking the check and moving the rook to safety (although white could play 2.Qxg7, taking the rook, black could then play 2. ...Kxg7, taking the queen; white will then have lost the Exchange and will be unable to destroy Black's pawn majority, resulting in a forced win for Black, a queened pawn, and a king-queen checkmate.)
However 1.Qd4 is by no means a bad move. Perhaps White played 1.Qd4 in order to force black to play 1. ...Rg7 in which White can now play 2.Qd8+ (fork on king and pawn) Rg8 (forced) followed by 3.Qxa5 in which case White can push his a-pawn. Here White forked Black's king and rook not to win the rook but to later win a pawn by a fork, and pave the way for his a-pawn to become promoted to a queen. He might have also intended to play 3.Qf6+ Rg7 (forced) 4.Qxc6, eliminating Black's passed pawn.
Pinned pieces 
A pinned piece is a piece that cannot move because it would expose an attack on an important piece by one of the opposing pieces, such that the capture of the important piece would result in material gain by the opponent. A very useful device is to pin the opponent's pieces to his king; this is known as an absolute pin. For example, imagine white's king on e1, a white knight on c3, and d2 empty. Black now moves his dark-squared bishop to b4. The white knight is now pinned and cannot move, because moving the knight would put himself in check, which is not allowed, of course. White's only way to stop the pin is to move his king. A pawn on e4 is no longer guarded by the knight, which could not capture a black piece taking this pawn.
In contrast to the absolute pin, a relative pin occurs when one player's piece is pinned to one of lesser value than the king, such as a queen or rook. If the benefit of moving the pinned piece outweighs the loss of material occasioned by the capture of the exposed piece (for example, if a forced mate may be achieved), then the pin can be disregarded and the pinned piece moved.
The position diagrammed to the right shows the theme of attacking the pinned piece. In this position it is white to move. The white player can look at this position and immediately take the rook and win an exchange. However if he did this he would not be playing the best move. If White did this then the material would be dead even; however, white has more pawn islands and it would be a very close game. White can instead play Ng5! The rook cannot move, and Black has no way of effectively defending the rook a second time in the next move, therefore in the end Black will lose a rook. So, if you have pinned a piece, take a look to see if you can attack the piece again, and if a piece of yours is pinned, take a look to see if your opponent can attack it again. And above all else, if you see a good move, stop and look around for the best move.
A skewer is similar to a pin, but it is in a sense more powerful. The difference between a pin and skewer is the location of the more important piece. In a pin the lesser piece blocks the attack on the more important one, in a skewer the more important piece is attacked first and when it moves it exposes the lesser piece behind it to be hit. Black has, in a blunderous moment, placed his king on d7 in front of his queen on d8. White may now triumphantly slide either rook to d1, skewering Black's king and queen. Since Black cannot block the check, the king has to move, exposing the black queen to the attack of the white rook.
A discovery is an attack on an enemy piece which is unveiled by moving one of your pieces. The power of discoveries is that two targets can be attacked simultaneously. If combined with a check they can be lethal.
Note that White's 1.Nd7+ was a double check, a type of discovered check where the piece moving also gives check. Such checks are very powerful, since the king is forced to move (both checks cannot be blocked at the same time). They are also often lethal, as in the above game.
A simpler example of a discovered checkmate is shown below.
Removing the defender 
By first capturing, threatening, or pinning a piece that guards another, you might be able to capture the other piece for free. You can also remove the defender of a position. By using tactics you might be able to force a checkmate!
A sacrifice is an exchange of a piece for a non material advantage:
In the first diagram, White just moved 1. Kh1 to get out of check.
Black sacrifices his queen with 1...Qg1+ for a winning positional advantage - White is in check and can not take with his king because the knight guards the queen.
2. Rxg1 - forced - this smothers the king - he cannot move because his own pieces are on every square he could go to - any check on an unguarded square now is mate.
2...Nf2# Checkmate. This queen's sacrifice was an example of a smothered mate (in which a knight delivers a checkmate that is caused by the king's inability to move anywhere). In fact, this is a special kind of smothered mate, called Philidor's Legacy.
A Zwischenzug (pronounced "TSVISH-en-tsook",) or "in-between move," is one that is made unexpectedly in the midst of a sequence of moves. But not just any series of moves, one in which the player falling for the Zwischenzug feels the sequence is forced, while his opponent demonstrates to him that it certainly isn't! Most commonly these fall in between trades where a recapture seems to be the only proper means of play.
Such in-between moves often have a surprising and pleasing effect of increasing the potency of a combination beyond the opponent's expectations.
Borisenkov-Mezenev (diagram, Black to play), went 1... f2, threatening to queen, which White countered with 2. Rg8, intending 3. Rf8+, and 4. Rxf1. But White resigned after the Zwischenzug 2... Bb1! which allows Black to queen (3. Kxb1 f1=Q+ or 3. Rf8+ Bf5).