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 can only play in a single major key and its relative minor. It can also play in what we call modes, but that is a subject for another day. An advanced bowed dulcimer player, however, can play in other keys by some smart string fingering, or stopping. Also, a diatonic bowed dulcimer can be retrofitted to become chromatic.

The modern bowed dulcimer is a lot different from its mountain namesake. To begin with, it is designed to be held upright, resting on the player's knees. Second, it has a feature common to all bowed instruments, a sound post. The sound post couples the top and bottom plates of a bowed instrument to transfer the vibrations of the sound plate to the bottom. The result is a stronger, fuller sound. It also has a bass bar under the soundboard, a feature also borrowed from the violin family. And it's most frequently a four-string chromatic. 

So now we have an instrument that is like a cello, but with frets. Hmm... That sounds pretty much like a viola-da-gamba, simply called a viol. Well then, we might as well also 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.

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.