A multiplexer (MUX) is a device allowing one or more low-speed analog or digital input signals to be selected, combined and transmitted at a higher speed on a single shared medium or within a single shared device. A MUX functions as a multiple-input, single-output switch. With two input signals and one output signal, the device is referred to as a 2-to-1 multiplexer; with four input signals it is a 4-to-1 multiplexer; etc.

A 4 to 1 line Multiplexer:
A 4-input multiplexer we would require two data select lines as 4-inputs represents 22 data control lines give a circuit with four inputs, I0, I1, I2, I3 and two data select lines A and B as shown below:

Circuit Diagram:

Truth Table: