In the 1920s and 1930s, some of the most talented linguists of the Soviet Union, among whom one can highlight N.F. Jakovlev and E.D. Polivanov, were involved in the process of "language building". Their role in the success of this process is examined from the point of view of the phonological theory that they developed for creating scripts for the numerous peoples of the Soviet Union, Turkic and Caucasian above all. Jakovlev's phonology, that Polivanov termed "social phonology", was very different from the one that N. Trubetskoj proposed some 10 years later. We will try to explain their ambitious script projects, which remain difficult to understand from the point of view of the modern phonology.