Multiplexers

The multiplexer (also known as a data selector) will select data from several transmission lines to be gated to the output transmission line.

The multiplexer will also have a number of control inputs that are used to select the appropriate data channel for input.
The number of data inputs is equal to 2^N where N is the number of control select leads.
The 74151 is a eight-line multiplexer with three select lines.
An additional input on the 74151 is an enable input which can be used to expand the size of the multiplexer.
A multiplexer can also be used to implement a SOP Boolean Expression in combinational logic.