There are four variables $$v_1$$, $$v_2$$, $$i_1$$ and $$i_2$$ in a two-port network as shown in the figure. Out of which, we can choose two variables as independent and another two variables as a dependent. So, we will get six possible pairs of equations. These equations represent the dependent variables in terms of independent variables. The coefficients of independent variables are called parameters. So, each pair of equations will give a set of four parameters.