Is There Continuity between Persian and Caspian? studies the south-central Alborz as a language transition zone. This mountainous area, also known as Qasran, consists of the upper valleys of the Jajrud and Karaj rivers, separated from both Tehran and Mazandaran by mountain chains, and of Shemiran, within Greater Tehran. There are dozens of villages in the area with vanishing dialects that show various degrees of affinity with the neighbouring languages. The following questions are addressed: Is there a sufficient amount of idiosyncrasy within Qasran to define it as a language group; how do the Qasran dialects relate to neighbouring Caspian, Persian, and Tati; and is there a language continuum or disruption? In addition to linguistic analysis, the study incorporates historical, socioeconomic, and emigrational data. The relationships among the dialects are categorized by making explicit the areal distribution of major linguistic differences in phonology, morphology, syntax, and lexicon. An analysis based on 48 isoglosses from 35 localities (amounting to more than 2000 linguistic items) is summarized in five bundle maps. These maps not only show some idiosyncrasy in the middle and southern part of Qasran but also reveal two distinct dialect groups, with thick isoglottic lines separating the dialects in the north and southeast—showing high degrees of affinity with Tabari (Mazandarani)—from the southern dialects, which are akin to Persian. This outcome dictated the coining of two dialect groups: Tabaroid and Perso-Tabaric.