Here all four parameters Z11,Z12 ,Z21 , and Z22 represent impedance. In particular, Z21 and Z12 are transfer impedances, defined as the ratio of a voltage V1(or V2) in one part of a network to a current I2(or I1 ) in another part . Z12 = V1 / I2 . Z is a 2 by 2 matrix containing all four parameters.