A musical instrument combining features found in the guitar and the harp. While there are several unrelated historical stringed instruments that have been referred to as 'harp-guitar' over the centuries, the most common definition is a "guitar, in any of its accepted forms, with any number of additional unstopped strings that can accommodate individual plucking". A harp guitar must have at least one unfretted string lying off the main fretboard, typically played as an open string. Originally manufactured as a novelty instrument, the harp strings are used as bass or drone strings, adding to the harmonic texture. Hybrid styles are possibly the rarest of all, although harp mandolins do exist. While most players of harp guitars play on acoustic instruments, a few of them also work with electric instruments.