The Slavic migrations divide the prehistory of the Slavic languages into two separate periods - a recent period which is fully accessible and amenable to the historical dialectologists methods, and an earlier period seemingly out of reach. Investigating a striking phonological irregularity of the modern Slavic languages which probably goes back to a clear-cut dialect difference in the ante-Migration Common Slavic speech area, this study undertakes the task of shedding light on the Slavic dialects that existed before the Slavic migrations. The author pays particular attention to questions of method, both concerning such general problems as the relation between assumptions, data and (hypothetical) conclusion and the more particular issues that arise out of the lack of temporal continuity between the modern dialects and the dailect divisions that may be discerned in the ante-migration Common Slavic Speech areas.