Bluegrass generates most of its interest from technical ability, even in its most traditional veins. Generally, the genre operates a lot like jazz: different configurations of instruments improvise solos on standard tunes. There’s mostly likely an upright bass and some light percussion like tambourine or washboard in the rhythm section, treble instruments like fiddle, banjo, mandolin, and guitar taking solos, and multi-part harmonies in the vocals. Bluegrass generally borrows from the same sources as country and folk: Scottish, Irish, and English folk music, African American spirituals, and blues. Progressive bluegrass started, like progressive rock, in the late 60s. While, the compositions never really reached the complexity of prog rock, the idea was the same in the beginning: the chord progressions got more complex, it started borrowing from other genres most notably jazz, modern rock, pop, and classical music, and the lyrics became deeper.