The most original part of Estonian folklore, the runic folksong, originated in the 1 st millennium BC, as a result of interaction between primitive Balto-Finnic singing (warbling, keening) and the songs of the Baltic tribes. Runic songs are based on the alternation of long and short syllables. The basic runic verse line is the trochaic tetrameter, with each line generally forming a conceptual whole. A song is constructed by the repetition of the concept of a line of verse in subsequent lines. The words are tied within the verse by alliteration and assonance. There are approximately 3,600 runic song types and 133,000 variations. The tunes have limited scope, but are rich in variations. The oldest are songs with myth themes, keenings, spells and family ballads; when agriculture became established, work and ritual songs with a magic function developed. Most of the surviving songs are from the serfdom period, when the singers were mainly women, and the content female-oriented. Due to the conditions of serfdom, the songs were mainly elegy-style and satirical, rarely is joy or love expressed.