Abstract
The article calls into question the traditional view that the merger of a and o took place as early as the Balto-Slavic or Early Proto-Slavic periods and was phonetically closer to a from the very beginning. The article tries to show that the explanation of the development of the Proto-Slavic vowel system is easier if we accept that the merger took place much later or that the merger first resulted in a vowel closer to o, and only later, at the beginning of the classical period, it was delabialized to a sound closer to a.