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This information might be useful:

Terz Pitch and Scale
The Terz guitar is tuned a minor third higher than a regular guitar, the same tuning as if you put a capo at the third fret of a normal guitar (e.g. open strings are G-C-F-A#-D-G). It has a much shorter scale length, typically 530-560mm, compared to the Torres 650mm scale, or even the period 625-635 scale. Italian Terz guitars were tiny in size, whereas Viennese Terz guitars adapted the Stauffer model, with its pronounced upper and lower bouts, to the smaller scale.
Tranposed Terz Music
Even though Terz guitar is tuned three notes higher, you play ordinary guitar music on it, except it sounds a third higher than written. Music written for normal pitch guitar may sound strange at the higher pitch of a terz guitar. Most people do not realize that a normal guitar in fact sounds an octave lower than the music is written. Because the Terz guitar sounds a third higher than the music is written, the guitar parts for duets with Terz and Normal guitar are written in different keys to account for the transposition. For example, if Guitar 1 - Terz is A major, Guitar 2 - Normal is written in C major. The Terz guitar in that case is actually playing in the key of C, although the music is written in A. Dr. Buckland was able to find several previously unknown Terz works by finding duets with mismatched keys, off by a third.
The advantages of a guitar with a root a third higher than a normal guitar with a capo are easy to see. A capo 3 reduces the number of frets available by 3, so that the highest note that can be played at the 12th fret is C# instead of E. The capo is confusing, since the frets are no longer at their original points, for example the 12th fret is no longer a point of reference, whereas a Terz guitar maintains the original frets. Modern editions of 19th century guitar music which claim it can be played with Capo 3 on a normal guitar are often not realistic, given the often frequent 12th position jumps and other idiomatic fingerings.
Terz Guitar Strings
The Romantic Terz Guitar was used for classical music and strung with gut strings, though today Nylon can be used instead. La Bella makes strings today specifically for classical / romantic Terz guitar, though I found their 2001L strings work fine as well. This web site has a section on Terz Guitar Gauges. DO NOT tune your normal guitar a third higher - this will cause too much tension and can damage your instrument. A terz guitar is 56-57cm scale, and even so, it still often requires lighter strings. M. Ophee sells string sets of very light gauge, so that you can string a 650 scale guitar as terz if needed - this is useful if you have an instrument not in service and need to play a couple of terz pieces but don't want to buy a terz guitar.

from https://www.earlyromanticguitar.com/erg/terz.htm