Weinstein analyzed the mode structures for open resonators that consist of a pair of ellipsoidal mirrors with a common principal section. 3 He did not get a closedform description of the transverse-mode distribution and no simple set of parameters can be used to characterize the output beam. However, because the effect of tilted components in a symmetrical spherical-mirror cavity may be accounted for by an equivalent cavity that has only ellipsoidal mirrors, we apply Weinstein's analysis to develop the concept of a mode envelope which can be precisely described by simple geometry.