A harp guitar is simply a guitar that has some extra strings that are not fretted. They’re generally meant to be plucked as open bass strings. Truth be told: a harp guitar is really not that similar to a harp.
How does one tune those extra strings? The simple answer is: however you want. Tunings vary from player to player, even from song to song. Some tune the extra strings diatonically; some chromatically; many players blend both approaches. In addition, different luthiers suggest different tunings. It’s a jungle out there.
One thing’s for sure: the harp guitar has a long history, reaching from the 1800’s to the present day. Some players have dedicated themselves wholly to the instrument; others have explored the harp guitar as an extension of their work on more standard instruments. That latter group includes guitar greats like Jimmy Page and jazz guitarist Pat Metheny.
If you’re interested in playing the harp guitar but you’re intimidated by the size of the instrument–in photographs it often looks like it’s eating its owner–you might try warming up with a harp ukulele first. Yes, they exist.
To read more about harp guitars, their tunings, and their most famous players, check out http://www.harpguitars.net/ and make sure that you don’t miss the section of documented harp guitarists throughout history! They are a colorful bunch…