We begin our study of tactics with double attacks, or forks: moves that attack two enemy targets at once.
And we begin our study of double attacks with knight forks. In the skeletal diagram to the left, White’s knight has forked Black’s king and rook; in other words, it attacks them at the same time.
Why start with the knight? Because it is an especially vicious and common forking tool. First, it can threaten a wide range of targets. The knight is roughly comparable in value to a bishop, and so is less valuable than a rook or queen; thus a knight not only can attack any unprotected (or “loose”) enemy pieces but also can be exchanged favorably for enemy queens and rooks regardless of whether they have protection. Second, the knight’s unique, non-straight pattern of movement creates two advantages: it allows a knight to attack other pieces without fear of being captured by them; and it enables a knight to make jumps and deliver threats that are surprising to the eye and so are easy to overlook.
To spot possible knight forks you will want to become habitually aware of the relationships between your knights and your opponent’s pieces (and between his knights and your pieces), especially as the knight progresses up the board. Every rank a knight moves forward tends to bring it closer to forking targets, especially the king; notice that once your knight reaches its fourth rank, it can attack your opponent’s back rank, and often his king, in one move (thus in the diagram to the left, White’s knight might have been on e4 a move earlier—seemingly pretty far from Black's king). Hence the strategic importance of planting knights on central and advanced squares, and the tactical importance of constantly looking for forks your knight might be able to deliver once it is properly developed.