Like the original Fender precision bass and the double bass, it has four strings, normally tuned to E-A-D-G, making it an octave below the lowest four strings of the 6-string guitar. Like the bass guitar, 5+ string models have been produced although these are relatively much less common. The physics of sound production means it is very hard to produce a useable low B string and five string varieties may be strung E-A-D-G-C instead.
There are two main varieties. The majority of acoustic bass guitars are fretted but a significant number are fretless instead. Semi-fretted versions are also occasionally seen.
Many but by no means all acoustic bass guitars are fitted with internal pickups, either magnetic or piezoelectric or both, and can optionally be used with an amplifier.
There are also semi-acoustic models fitted with pickups and intended to be always used with an amplifier. The box of these is principally designed to produce a distinctive tone when amplified, similarly to semi-acoustic electric guitars. Thin-body semi-acoustic basses such as the violin-shaped Hohner made famous by the early Beatles and several Fender models are not normally regarded as acoustic basses at all, but rather as hollow-bodied electric basses. As with semi-acoustic electric guitars, the line between acoustic instruments fitted with pickups and electric instruments with tone-enhancing bodies is sometimes hard to draw.
Other manufacturers, not mentioned above, include Eston, Gibson, Maton, Prestige and Tacoma.