The absolutely basic bowed dulcimer is a diatonic instrument with three strings tuned to DAD, following essentially the stringing, fretting and tuning of a mountain dulcimer. A baritone bowed dulcimer is most frequently tuned to ADA or GCG.
A diatonic instrument is confined to play in a single major key and its relative minor, and you would expect the same with the basic bowed dulcimer. An advanced bowed dulcimer player, however, can bypass the problem of missing frets by some smart string fingering, or stopping.
The modern bowed dulcimer is a lot different from its mountain namesake. To begin with, it has a more comfortable shape so that it can be held upright resting on the player's knees. Second, it has a feature shared with all bowed instruments, a sound post. The sound post couples the top and bottom plates of the bowed instrument to transfer the vibrations of the sound plate to the bottom. The result is a stronger sound. It also has a bass bar under the soundboard, a feature also borrowed from the violin family. And it's almost always chromatic and almost always stringed with four strings.
So now we have an instrument that is like a cello, but with frets. Hmm... That sounds awfully close to the viola-da-gamba, a four string bowed instrument with a bass bar, a sound post and a fretted fingerboard. So, we might as well call the bowed dulcimer a viol, and make it a legitimate relative of all the other instruments in the bowed fiddle family, instead of a poor orphan that's neither a dulcimer nor a violin.
Be that as it may, the four-string bowed dulcimer allows several tunings: the viola-da-gamba tuning of DGBE, similar to the upper four strings of the guitar; also tunings based on perfect fifths like orchestral stringed instruments. The two most popular of these tunings from low to high are CGDA and GDAE. The CGDA tuning is the same as the viola (no relation to the viola-da-gamba), or an octave higher than the cello.
The GDAE tuning is an octave lower than the violin. The two tunings overlap on the "D" and "A" strings, which are the same for both.
A bowed dulcimer stringed for viola tuning can be re-tuned as a viola-da-gamba without having to replace any strings. But to change from viola to octave violin or vice-versa, the instrument must be re-stringed with strings of appropriate gauge.
Some older bowed dulcimers followed the stringing orientation of the mountain dulcimer, with the highest string closer to the player. Because of the positioning of the bass bar and the sound post, stringing orientation plots are not reversible.
For an article on Bowed Dulcimer tunings click here. And for instructions on how to bow your mountain dulcimer, click here.