It has either 10 or 12 strings. With them, it’s possible to play a broader range than either the standard guitar or bass can do individually. Author Dave Hunter says that Emmett Chapman invented it in the 1970s to offer jazz players more options:
Its range runs the gamut from the bass’ low to the guitar’s high. Traditional tuning, if you can call it that, goes low E, A, D, G, C, although the bass strings progress upward—that is, the reverse of those on a standard bass guitar, from highest pitched string at the top to the lowest pitched at the middle of the tapboard; the treble strings run conventionally downward, tuned F#, B, E, A, D.
-via Twisted Sifter