The northeastern Lithuanian vocal sutartines feature a great diversity in their polyphonic structure: they vary in texture (polyphonic and heterophonic), modal structure (polymodal and monomodal), and rhythm (polyrhythmic and homorhythmic). Since the prevailing melodic style of sutartines is mixed rather than homogeneous, it is probable that these polyphonic songs originally evolved through the interaction of different folk music styles. Most of polyphonic sutartines, according to Latvian and Lithuanian ethnomusicologists, were composed by juxtaposing monophonic melodies. This was proved by comparing the melodic structure of sutartines with that of the monophonic melodies from the adjacent regions.Such characteristics of sutartines as polymodality, cannot be fully explained in regard to the development of vocal melodies. This leads to an admission that vocal sutartines underwent a considerable melodic influence of instrumental music. The polyphonic polymodal sutartines have probably derived from the process of playing the two daudytes rather than singing. The polymodality could have resulted from the performance on two daudytes of different length. Historical sources provide us with evidence that earlier the tradition of performing daudytes was highly significant. Thus it appears that the influence of instrumental music on modal structure of vocal sutartines was much more considerable than it was generally thought.