Common bass tunings are fifth bass and the older octave bass.
Common sizes are 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.
A typical fifth bass arrangement of notes would be as in the following
diagrams. The heavy diagonal lines indicate treble and bass bridges.
The treble bridge is on the left. The horizontal lines indicate
strings (or courses of multiple strings per course). The hollow
squares are strings (or courses) crossing a bridge at a marker.
The octave bass arrangement is
shown only for a 12/11. The strings on the bass bridge are tuned
an octave below the strings on the right side of the treble bridge
directly across (and slightly above) them.
The D in the second octave of the bass bridge is in unison with the low D on the treble bridge. From there on up, strict octaves would duplicate notes on the treble bridge, so some of the accidentals missing on the treble bridge may be located here, as the F natural and B flat are in the diagram.
If you look for chords in either diagram, say a G major chord (G-B-D), you will notice that notes fall into patterns, such as triangles, or ever other string in a line. For more information and exercises on playing with patterns, see the Square One books for hammered dulcimer, from which these diagrams were taken.