The lecture centres on changes in the system of unstressed vocalism that led to the development of akanje/yakanje, yekanie, and ikanye (neutralization of unstressed vowels in certain positions) in some Russian sub-dialects. The focus of attention is the development of akanje/yakanje and its various types. Different points of view on the origins and development of akanje are offered in the lecture. Special attention is given to representation of akanje/yakanje in written sources. In conclusion, some general comments on the development of Russian sound system and its results are offered to the students.