The paper investigates the geographical denominations of Western lranian dialects, largely functioning also in the place-names, hydronyms, and oronyms of the area. The relevant lexical material, when taken together and approached as a system, opens larger possibilities for adequate analysis: the paradigm and internal ties of the constituent units become more visible and more clear revealing many otherwise unseen tendencies and peculiarities, particularly regarding the origin of given terms or groups of lexemes within the system. Moreover, as a mirror, it can detect various areal characteristics - first of all lexical and phonetic - of a language or language group in a particular territory. ln other words, it can reveal not only the features of linguistic development in diachrony, i.e. in time, but also in diatopy, i.e. in space.