D Bass string
1. Tune the bass string to low D. You can find it on the piano by counting down 6 white keys from middle C.
A Middle string
D Treble string (2 strings)
2. Tune the middle string to A. Count down 2 white keys from middle C on the piano.
3. Tune the treble string to D. Count up 1 white key from middle C or tune it the same as the bass string but an octave higher.
You can also tune the dulcimer to itself by tuning the bass string to D or a nice low tone then fret the bass string at the 4th fret and tune the open middle string to that pitch. Then fret the middle string at the third fret and tune the open treble strings to that pitch.