The Triolian started out in the late '20's as a lower-priced wooden guitar.but with the same "tricone" system (see Mandroid's post above) as the first Nationals.I don't think too many people play them as a main instrument more like a "fun" instrument or one for the collection like Banjolin and emando.I bought one when I saw the Sam Bush in concert and he floored the audience with his playing slide blues on it.The National is brash, aggressive, louder, with more of a "bark." There are many other differences.Many National mandolins were made of metal, either steel or plated brass. National mandolins tend to have a longer scale, and often National owners tune them as mandolas, or at least at lower pitch than the standard GDAE.The National's resonator cone, also aluminum, rises up in the center, the opposite of the Dobro's cone.The highest point in the center of the National cone has a circular wooden "biscuit" glued to it, to which the bridge is attached.
Only Duolian guitars were made, in 12-fret models up to 1935, afterwards (until 1940) 14-fret.There's no intervening "spider" structure, and the strings' vibration is transferred directly to the National's cone through the "biscuit bridge." The sound difference is readily apparent.The Dobro is softer, with a smooth "ringing" sound and a long sustain.Attached to it is an aluminum "spider," with legs that radiate to the edges of the cone; in the center of the spider is a wooden bridge over which the strings pass.The bridge transfers the strings' vibration to the spider, which in turn vibrates the cone, which makes the sound.