The 6-string banjo began as a British innovation in the 1860s, originally marketed as a 'zither banjo'. This usually had a closed back and sides with the drum body and skin tensioning system suspended inside the wooden rim, the neck and string tailpiece mounted on the outside of the rim, and the drone string led through a tube in the neck so that the tuning peg could be mounted on the head. They were often made by builders who used guitar tuners that came in sets of three, so 5-string instruments had a redundant tuner and could thus be somewhat easily converted to a 6-string instrument. 

Modern 6-string bluegrass banjos have been made. These add a bass string between the lowest string and the drone string on a 5-string banjo, and are usually tuned G4 G2 D3 G3 B3 D4.