I propose that the ubiquitous 12-string guitar may have largely evolved from and been inspired by two previously overlooked instruments: An early non-floating-strings instrument named the “harp-guitar,” and the true harp guitar with floating strings.
link
The Birth of the American 12-string Guitar
Harpguitars.net
The Birth of the American 12-string Guitar
Gregg Miner traces the American 12-string guitar's origins to Carl Brown's 10-string harp-guitar, René Grunewald, and early doubled-course harp guitars.