Basically in digital system there are two type of circuit: combinational logic circuit and sequential logic circuit. When the authors talk about combinational circuit, this circuit is that circuit of which output depends on input like half adder, full adder and in the sequential circuit output is depend on present state like flip flop etc. In digital circuit addition process is biggest and very important process. With the help of this method digital equipment perform various types of operation such as addition multiplication etc. on the basis of working principle there are two types of adder: serial adder and parallel adder.