Adders are the combinatorial circuits which are used to add two binary numbers. The nature of the adders chosen depends on the characteristics of the binary numbers which need to be added. Say for example, if one needs to add two single bit binary digits, then one can use half adder while if there is an additional carry which needs to be added along with them, then one may resort to the use of full adder. However what if we want to add a binary number which has multiple bits in it. In such a case, the need arises to use a parallel adder.

Parallel adder is nothing but a cascade of several full adders. The number of full adders used will depend on the number of bits in the binary digits which require to be added.
Such a n-bit adder formed by cascading n full adders (FA1 to FAn) is as shown by Figure 1 and is used to add two n-bit binary numbers . Here, every single bit of the numbers to be added is provided at the input pins of every single full adder. That is, the first bits A1 and B1 are provided as the inputs to full adder (FA1), the second bits A2 and B2 to the inputs of full adder 2 (FA2)… and the last bits An and Bn to the nth full adder FAn. Next, the carry out pin of each full adder in the circuit is connected to the carry in pin of its succeeding full adder (except in the case of last full adder). For example, the carry out pin of FA1 (Co1) is connected to carry in pin of FA2 (Ci2), the carry out pin of FA2 (Co2) is connected to carry in pin of FA3 (Ci3) and so on and so forth. 