Unlike the electric bass guitar, which is generally a solid body instrument, the acoustic bass guitar usually has a hollow wooden body similar to (though usually somewhat larger than) that of the steel-string acoustic guitar. The majority of acoustic basses are fretted, but a significant number are fret less instead. Semi-fretted versions also exist, although they are quite rare.
Like the traditional electric bass and the double bass, the acoustic bass guitar commonly has four strings, which are normally tuned E-A-D-G, an octave below the lowest four strings of the 6-string guitar. Like the electric bass guitar, models with five or more strings have been produced, although these are less common. In part, this is because the body of an acoustic bass guitar is too small to produce a resonance of acceptable volume at lower pitches on the low "B" string. One solution uses the five string acoustic bass to add an additional high string ("E-A-D-G-C") instead of adding a low "B". Another solution is to rely on amplification to reproduce the low "B" string's notes.
There are also semi-acoustic models fitted with pickups that are intended to be used with an amplifier. The soundbox of these instruments is not large enough to amplify the sound; instead, it is designed to produce a distinctive tone when amplified, similarly to semi-acoustic electric guitars. Thin-body semi-acoustic basses such as the violin-shaped Hofner made famous by the early Beatles and several Fender models are not normally regarded as acoustic basses at all, but rather as hollow-bodied bass guitars. There are also semi-acoustic basses such as Godin Guitars' "A-Series" that, once amplified, sound much closer between acoustic bass guitars and upright basses, and have been used in professional circles to "simulate" one when it would be impractical for transportation and other reasons to use a full-sized upright bass.
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, especially when some instruments can also be equipped with a variety of pickups such as piezo pickups, the "standard" of acoustic-electric instruments as well as synth pickups which can replay "virtual" upright bass sounds and bring a semi-acoustic bass much closer to a double bass sonically. Saga Musical Instruments produces a four-string bass resonator guitar under their Regal brand name. National Reso-Phonic Guitars also produce three models of resonator bass guitar.
