The strings are arranged in unison pairs or triplets, called "courses". The treble courses run from hitch pins in one pin block, over the treble bridge and under the bass bridge, to tuning pins on the other pin block.* The bass courses run over the bass bridge and under the treble bridge. The treble bridge is arranged so that each portion of a treble string course to the left of the treble bridge is a perfect fifth higher than the portion to the right, so the treble courses are playable on both sides of the treble bridge. Usually the bass strings are playable only on the left side of the bass bridge.
* (Some builders may put hitch pins and tuning pins on both sides; and some use another set of tuning pins as hitch pins, for tuning from either side.)
This string and bridge arrangement allows a considerable range in a compact space. It also leads to severe visual disorientation if one focuses on the strings instead of the bridges. The visual disorientation is even worse when bright lighting casts shadows of strings on the soundboard.
The treble strings are tuned in diatonic (do re mi) scales in groups of four courses. For example, from the lowest of the white position markers to the next marker, the treble courses on the right of the treble bridge would sound D, E, F#, G; and on the left (a fifth higher) A, B, C#, D'. From the second postion marker to the third, we would have G, A, B, C on the right and D, E, F#, G' on the left. And so on. See the Diagram.
Common bass tunings are fifth bass and the older octave bass. In fifth bass tunings, the bass string course immediately below (and to the right) of a right hand treble course is tuned a fifth lower. For example, a G would be the bass string note next to the treble string D. In octave bass tunings, it is one octave lower. Extra strings, and even extra bridges are used to add notes missing from the diatonic scales. Extra octaves may also be added at the top or bottom. In current parlance, the size and range of a dulcimer is indicated by what looks like a fraction: "12/11" or "15/14". The first number in a pair refers to the number of treble courses; the second to the number of bass courses. The tuning description above with a D scale starting at the lowest position marker is typical for a 12/11. On a typical 15/14, the extra course is an A scale, below the D course. Diagram
The tuning arrangement is "left handed" compared to the dulcimer's daughter, the pianoforte. On the dulcimer, lower pitches are to the right; on the piano the higher pitches are to the right.