Reciprocality Theorem

The reciprocality theorem tells us that in a linear passive bilateral network an excitation and the corresponding response may be interchanged. 

In a two port network, if an excitation e(t) at port (1) produces a certain response r(t) at a port (2), then if the same excitation e(t) is applied instead to port (2), then the same response r(t) would occur at the other port (1).