Hammered dulcimer is the forerunner of the piano. Very ancient (5000 years?), has a whole bunch of strings strung across a trapezoidal shaped body. You hit the strings with sticks to get the sound. Mountain dulcimer AKA Appalachian dulcimer AKA lap dulcimer is 200-300 years old, originated in Appalachian Mts (USA), has usually 4 strings. You hold it on your lap and strum across the strings. Two are drone, two are close together and are the melody strings. It has frets corresponding to the scale (not chromatic like a guitar). Body is usually hourglass shaped, sometimes teardrop shaped, sometimes other. No neck, just a peghead.