Bluegrass duo is a term used to describe a musical group consisting of two members who play bluegrass music. The term 'ESSES' is an acronym for the two members of the bluegrass duo, which stands for 'Electric Stringed Stringed Instrument and Stand-up Bass'. This type of musical group is typically composed of a guitar player and a bass player, both playing acoustic instruments. The combination of the two instruments creates a unique sound that is often associated with bluegrass music.