The defining equations for V1 and V2 analyzing a two-port network in terms of its impedance parameters are respectively
1
Z12I1 + Z12I2 and Z21I1 + Z21I2
2
Z11I1 + Z12I2 and Z21I1 + Z22I2
3
Z21I1 + Z21I2 and Z12I1 + Z12I2
4
Z12I1 + Z12I2 and Z21I1 + Z22I2