The theorem is stated as follows :

“In any linear bilateral network, if a source of e. m.f E in any branch produces a current I in any other branch, then the same e.m.f acting in the second branch would produce the same current I in the first branch”.

In other words, it simply means that E and I are mutually transferrable. The ratio E / I is known as the transfer resistance (or impedance in a.c. systems).