ENThis article deals with a hitherto unexplained variation of short and long variant forms of the third person subjunctive in Lithuanian dialects from a comparative perspective: many older (and younger) dialect records attest the long 3subj. form 'būtų' ‘be’ alongside the short 3subj. form 'būt' without '-ų'. It seems that this variation also recurs in the nominal domain in gen.pl. 'mūsų', 'jūsų' vs. 'mūs', 'jūs'. The article tries to work out the original distribution pattern of these forms and attempts to explain how they came into being. Comparative evidence from Lithuanian dialects and Latvian indicates that the distribution of the forms was originally dependent on prosodic factors: long forms were prosodically independent default variants, while short forms were contextual variants that occurred as the last element of a complex intonation unit. The systematic comparison of relevant Lithuanian and Latvian data leads to the conclusion that the variation of the long and short forms is already of Proto-East-Baltic date, and motivates further comparative research into Baltic prosody. Keywords: Lithuanian dialects, Latvian dialects, East Baltic, subjunctive, prosody, historical linguistics, diachrony. [From the publication]