A large Mexican fretless 6-string bass guitar with a heavily vaulted (or arched) sound box, which functions as the lowest voice in mariachi music. Tuned ADGCEA, the strings are often plucked in pairs to provide octaves.

Although similar to the guitar, it is not a derivative of that instrument, but was independently developed from the sixteenth-century Spanish bajo de uña. It achieves audibility by its great size, and does not require electric amplification for performances in small venues.