Musical Encyclopaedia(Russian) said that bal. mainly tunes in major chord(triad), rarely upper string tunes in fourth with two bottom strings which tunes in unison. My friend Dima tunes b. in this way: two upper string in unison, bottom one in fourth with them. I think b. like all other folk instr. tunes in many way. You can play melody, intervals, or chords. I heard many virtuos b. players. They play b. solo and accomp. theyselves when singing. Best wishes!