Edited sentence: A common simple topology that cannot be solved by series and parallel combinations is the input impedance to a bridge network (except in the special case when the bridge is in balance).In the general case additional transformations are required.Its importance arises from the fact that the total impedance between two terminals cannot be determined solely by calculating series and parallel combinations except for a certain restricted class of network.The Y-Δ transform, or its inverse the Δ-Y transform, represents the minimal additional transform required to solve the general case.This is a particularly important transform for finding equivalent impedances