Skip to: Harp Guitar Tuning Reference Chart
(includes Modern Tunings)

Probably the first official stab at a Tuning Chart for harp guitars was John Doan's contribution in his 1988 Frets article.

Other than that, most players simply come up with their own best guesses or inventions.
For example, John created his own now-standard "20-string" tuning, while Stephen Bennett's ingenious variation of tuning the first sub-bass to the neck's third fret G has caught on with a number of players.

This page will present both modern and historical tunings - though it will not (for now) go into super-trebles, nor the endless open and altered tunings of contemporary players.

So our focus is: How do we tune those sub-bass strings?

In general, sub-basses descend in pitch away from the low E string on the neck.  
Occasionally,  the first 1-4 basses are tuned higher than the low E (Stephen Bennett, Gibson).  
In general, sub-basses from 1 to 6 in number are tuned diatonically, while 7 to 12 sub-basses are tuned chromatically.
Within the typical 5 to 7 sub-bass bank that modern players prefer, an endless variety of altered tunings are possible, just as on the neck.  

Historical tunings are not consistent either, and manufacturers will even change their own suggested tunings (see Gibson: Appendix).  Even the "traditional" tuning of the Viennese bass-guitars or contra-guitars experienced many local variants.  As an example, in 2006, an 1899 German Zimmermann catalog provided new, very unique "historical" tunings that none of us would have ever suspected, as shown in the following table:

This first example from a c.1900 Zimmermann catalog includes an error in having the basses marked with an "x" indicating they are pitched an octave lower, which is not the case. These are all "re-entrant" tunings, in that the sub-bass strings start significantly higher than the lowest string on the neck.  At first I thought these were impossibly low bass tunings, as indicated by the erroneous transcription - Zimmermann is clearly stating that notes marked with an "x" sound TWO octaves lower, while the fretted strings without the "x" sound only one octave lower (please note that "h" = the pitch B).  However, Oleg Timofeyev and John Schneiderman pointed out the error.  Istead, ignore the "x" and take the notation as it stands (all sounding an octave lower than written, as standard for guitar).  The intent is to provide the missing tonic notes, excluding those on the lowest two fretted strings.

(image courtesy of Christian Steinbrecher)

By comparison, other catalogs include more "traditional" diatonic and chromatic tunings: