Baritones are larger mountain or appalachian dulcimers with 29 1/2" scales, typically tuned AEA' or GDG' below a standard dulcimer, with a correspondingly bigger, deeper sound - really majestic on slow tunes, and ballsy like an Irish octave mandolin on flat-picked reels and jigs. A true baritone is not just a standard dulcimer with heavy strings - you really need the longer string length, bigger body, and altered bracing and bridge to optimize the sound. They come in 3, 4, 6, and 8 string varieties, with average dimensions about 2" longer than a standard dulcimer. With a 1 1/2 fret they also have a larger variety of chords for backup work than a typical DAD' instrument. An interesting equidistant 4 or 8 string option is ADAD and variations tuning (from low to high) that gives you more range, mirroring an octave mandolin - a baritone and standard combined in one instrument, with the ability to remove one outside course or the other, if you wish to play only 3 at a time. I can also build DAD' instruments with this scale length for a fuller sound and more sustain.
Above: Baritone Dulcimer by our endorser Ron Ewing dulcimers. As you can appreciate, it has some extra frets that are not commonly found in standard mountain dulcimers, Ron installed the half and one and a half fret and this has brought a wealth of new possibilities making it more chromatic.
I'm the happy owner of one Baritone dulcimer myself. Videos and audio samples coming soon, though now I leave you with a video from a more accomplished player than I am or will be for a while.