In recent years I have done researches in the Palóc dialect area. In this region the diphthongs were used by the speakers of a larger area in earlier eras of Hungarian language history. In some dialect groups and local dialects of this region even in the 19th century also reported diphthong pronunciation. At the end of the 20th century and now most of the Palóc speakers do not pronounce diphthongs, this data is demonstrated in language tests. It can be detected divergent linguistic movements. We can perceive those different phenomena on both sides of the Trianon border in the earlier Nógrád county (the borderline between Hungary and Slovakia is here at the present). The geographical boundary line marking the area in which a distinctive linguistic feature commonly occurs. The isogloss means a geographical dialect continuum. In my study, I try to draw the temporal and spatial boundaries of diphthong use and disappearance. Sometimes these differences will be larger, sometimes smaller, but they will be cumulative. My corpus shows linguistic changes from the Middle Age to the present. My analysis was done by the following written and oral data: 1. The Code of Gömöry from the 16th century – the nun who copied it (her name: soror Katherina, Legéndy Kató), comes from the Palóc dialect region from the village Legénd. My resources contain the descriptions of the speech and customs of the Palóc speakers from the 18th and the 19th century (Matthias Bel 1735, from the northern Palóc region; Fábián Szeder 1819, the first and the following researches in the area along the river Ipoly; Imre Hollók 1836, from the region of Gömör; István Szabó 1837, in the valley of Karancs; Antal Reguly 1857, notes on her trip in the land of Palóc ethnicity; Gyula Pap 1865, from the region of Salgó; Sándor Pintér 1880, her collections in the western central region of the Palóc; Gyula Istvánffy 1890–1900, her collection of the folk poetries in the area of Mátra); 3. Atlas of the Hungarian Dialects from the middle of the 20th century; 4. the data from the atlases of dialect islands from Slavonien (at the present: Croatia); 5. Dialect Atlas of Medvesalja; 6. and language databases from the 20th and 21th century, my own research from a village Karancslapujtő in the Palóc area. Based on the data, I try to present the isogloss of the spread of diphthongs in space and time in the Palóc dialect area groups.