This paper sets forth a new theory for the origin of the ancient Greek class of place-noun derivatives characterized by a stem formant that appears as either -ών- or -εών- depending on the dialect. In the classical period and afterward, the stem formant acts as a simple productive suffix that derives place-nouns from noun bases. I propose that these place-nouns were originally formed as further derivatives of derived adjectival bases. Later, but still at a relatively early stage of Greek, the combination of the genitival suffix -ε(ι)ο- and the substantivizing ‘Strabon suffix’ - ō̆n- was reanalyzed as monomorphemic and propagated as a productive unitary formant.