Here’s a cool old patent from 135 years ago, showing a double-necked dulcimer with a rectangular body. These “double” dulcimers were meant to be played by two people at once, usually seated on either side of a table.
It is interesting that the inventor does not actually refer to this instrument as a “dulcimer”, but rather as being like a zither. So this must pre-date beginning to call this style of instrument a dulcimer or Appalachian dulcimer.
These were also sometimes known as courting dulcimers, because when a young man came calling on his sweetheart, her parents would seat them at the table with one of these in between. The parents could then leave the room, and as long as they heard both sides of the instrument being played, they’d know that the young hands were not busy elsewhere.
You can see in the patent’s drawings that the fretboards also were marked with shapes, which harken back to an earlier form of musical notation that used shaped note heads to help indicate pitches.
This may not be the earliest dulcimer patent, but it stood out as being of interest to homemade instrument builders.