The diffuse ionospheric resonances Dn, stimulated by topside sounders, have been studied for over 2 decades. These resonances are observed below the plasma frequency ƒN between the harmonics of the electron gyrofrequency ƒH. The Dn resonances are often accompanied by upper and lower subsidiary branches. The present paper concentrates on the classification of diffuse resonances, one motivation being the possible application to the interpretation of naturally occurring radio emissions in the magnetosphere. Osherovich has shown that the Dn resonances are characterized by a nonequally spaced spectrum of frequencies ƒDn = ƒDsn1/2 (n = 1, 2, 3, and 4), where ƒDs = 0.95(ƒNƒH)1/2, and that the upper subsidiary resonances Dn+ can be considered as upper hybrid resonances of Dn with frequencies given by ƒDn+ = (ƒDn² + ƒH²)1/2. This classification is here extended to include the lower subsidiary resonances Dn− (n = 1, 2, 3, and 4), and it is shown that their frequencies are related to ƒDn and ƒH by the expression ƒDn− = (ƒDn² − ƒH²)1/2. This result is based on a combination of previously published data and newly scaled ionograms from the Alouette 2 and ISIS 1 topside sounder experiments.