A pair of terminals, which are external terminal points for currents to enter or leave a circuit, is called a port. Elements, such as a resistor with two terminals, are one-port circuit. A two-port circuit has two separate ports for input and output. Several currents flow through circuit elements in a circuit and, consequently, voltages appear at their terminals. But, not all of them are interest in applications. Modeling a circuit by a two-port circuit reduces the current variables to two. The two currents and the corresponding voltages are related by nodal and loop analysis. A two-port circuit may have dependent sources but no independent source.