The author deals with a central question for the ethnomusicology of the Orient: the ‘neutral tone’ also imprecisely called ‘quarter-tone’. His article reviews the evolution of musical intervals in several traditions of the Turkic and Persian worlds since the early twentieth century. Analysing differences between nomadic and sedentary societies, J.D. searches an explanation to the absence of neutral tone in the classical musical tradition of Turkistan and, more generally, in the traditions of Turkic peoples. Based on a precise and complex musicological work of intervals measures, enriched with references to ancient recordings, with pictures and fieldwork material, this study shows how one distinguishes “two trans-national cultural areas” in regard to the different perceptions of intervals by, respectively, Turks and Persians, and to their building of music through notes progression. In regard to these facts, the author also examines how these modal traditions have evolved till the present, ending with the example of the Azerbaijani muqam.