Since the parameters Z are obtained by opening the input or output port circuitry, they are known as Open Circuit Impedance Parameters and its unit of measure is OHM.

We can define the parameters as:

a) Z₁₁ ⇒ Open circuit input impedance.

b) Z₁₂ ⇒ Open circuit transfer impedance from Port 1 to Port 2.

c) Z₂₁ ⇒ Open circuit transfer impedance from Port 2 to Port 1.

d) Z₂₂ ⇒ Open circuit output impedance.

We calculate Z₁₁ and Z₂₁ connecting a voltage source V₁ or a current source I₁to Port 1 while we left Port 2 as an open circuit, that is,I₂ = 0.Thus, knowing V₁ , I₁ and V₂ we can determine these parameters.

In the same way, we calculate Z₁₂ and Z₂₂ connecting a voltage source V₂ or a current source I₂ to Port 2     while we left Port 1 as an open circuit,     that is, I₁ = 0.

When Z₁₁ = Z₂₂ we say that the circuit is SYMMETRIC. This it means that we can divide it into two similar halves.

When Z₁₂ = Z₂₁, we say that the circuit is PASSIVE or RECIPROCAL. This means that the quadripole is linear and has no voltage sources DEPENDENTS.

See the Figure 31-02 a model of an equivalent circuit T, only valid for circuits Reciprocal or Passive.

Note that if we want to calculate Z₁₁ we have to put a source of voltage on Port 1, in this case, represented by V₁ and leave open the Port 2, that is, I₂ = 0. This means that we do not have the impedance Z₂₂ - Z₁₂ (represented in the color "pink") in the circuit. Then we have to add the impedances represented by the "blue" and "yellow" part and this neutralizes the impedance Z₁₂ left alone Z₁₁.

The same reasoning can be applied to calculate the other impedances. Thus, our model achieves the objectives.

If the circuit is not Reciprocal or Passive, that is, have dependent sources, then we have to modify the model. See the Figure 31-03 for an equivalent circuit model for general cases.

Notice that this circuit is derived directly from the equations of the Parameters Z given at the top of the page.

It should be noted that not always the Parameters Z can be described by the equations. As an example, we have the ideal transformers.

There is a relation between the parameters Z and Y given by the equations: