To realize a 4-bit full adders, you have to put together the partial solutions. In order to simplify this you use a trick called "black box".

A
With this black box you can now easily draw a full |

The **carry-out** bit of one digit becomes the
**carry-in** bit to the next level digit.
At the end of the calculation you get the result of the 4-bit addition
plus one carry bit.
This arrangement can be continued, so you can realize effortlessly
8 -, 16 -, 32 - or 64-bit full adders.

With the IC's that you bought, you can no longer build this circuit.
The circuit, built up only with basic gates,
would be very complicated and messy.
Therefore, IC's are available containig a full adder as a
"black box". A black box is therefore not just a trick on paper.
In digital electronics, it is often used to **simplify circuits**.

But now you get to know one other important field where logic gates are used.