“I have not added four strings to the guitar out of a whim, but out of necessity. The strings that I have added incorporate all the natural [sympathetic] resonance that the instrument lacked in eight* of the twelve notes of the equal tempered scale.” (Narciso Yepes. Ser instrumento. Speech of Ingression into the Real Academia de Bellas Artes de San Fernando, delivered on 30 April 1989.)
*The eight notes to which Yepes refers are: 1) C, 2) C#/D-flat, 3) D#/E-flat, 4) F, 5) F#/G-flat, 6) G, 7) G#/A-flat, 8) A#/B-flat.
“In the first place, the four supplementary strings [C2, Bb2, G#2, F#2] give it a balanced sound which the six-string guitar is far from having. In fact, at the moment of playing a note on one string, another begins to vibrate by sympathetic resonance. On the six- string guitar this phenomenon is produced only on four notes [E, B, A and D], while on mine the twelve notes of the scale each have their sympathetic resonance. Thus the lopsided sonority of the six-string guitar is transformed into a wider and equal sonority on a ten-string guitar. Secondly, I do not content myself with letting the extra strings vibrate passively in sympathy; I use them, I play them according to the demands of the music to be interpreted. I can control the volume of the resonances, or I can suppress them. I can damp one if it is inconvenient in a given passage, but if I can do this it is precisely because I have these resonances available.” (Narciso Yepes. The Ten-String Guitar. Trans. Lionel Salter. July 1973.)
If this explanation is still unclear, see: https://www.youtube.com/watch?v=jwLWmhGrdBU
“Many people have said to me that this is the same principle as that used for the viola d’amore … with seven strings that were mounted underneath the normal ones and vibrated in sympathy. But there was a problem with that instrument: The tuning – of both the bowed strings above and the sympathetic strings below – was D, A, F, D, A, F, D … Thus when you played a D you had not only the sound of that one string, but also the sound of all the other D’s on the instrument, so you had a very big D ! But, when you played G , for example, you had absolutely nothing in the way of resonance. My idea of the 10-string guitar is exactly the contrary – to provide sympathetic vibration for the notes that do not have this kind of reinforcement on a normal 6-string guitar.” (Narciso Yepes. 1978. “The Ten-String Guitar: Overcoming the Limitations of Six Strings”. Interviewed by L. Snitzler. Guitar Player 12, p. 46.)
The 10-stringed harp guitar is “Not exactly the same, because the tuning that I use is also for the resonance”. (Narciso Yepes, in Schneider, J. 1983. “Conversation with Narciso Yepes”. Soundboard, Spring: pp. 66-68.)
‘Another reason for the 10-string is that guitarists are always playing music written for the Renaissance or the Baroque lute. We can say that the lute is to the guitar as the harpsichord is to the piano. And if this is true, how can we take the music written for these eight, nine, or 10-course instruments – even [eleven,] thirteen and fourteen courses, in the case of the baroque lute – and transcribe it for a guitar, which has only six strings? [...] I want to be able to make “legitimate” transcriptions in which the music loses nothing, but rather improves in quality.’ (Narciso Yepes. 1978. “The Ten-String Guitar: Overcoming the Limitations of Six Strings”. Interviewed by L. Snitzler. Guitar Player 12, p. 26.)
“With the ten-string guitar I have many possibilities, and I do not need the baroque [lute] tuning exactly.” (Narciso Yepes. 1983. “Conversation with Narciso Yepes”. Interviewed by J. Schneider. Soundboard , Spring: p. 66.)
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For 11-course baroque lute music, raise the 7th string to D. Apply additional scordatura (mis-tuning) according to the lute’s re-tuning in a particular composition.
For 13-course baroque lute music, lower the 7th string to B/B-flat (or retain C, in some cases, if that bass predominates); fret additional low basses on the 7th string. Apply additional scordatura according to the lute’s re-tuning for a particular composition, or as otherwise required.
In order to translate to the 10-string guitar most of the lute idioms (e.g., ligatures, campanella effects, open strings): For both 11- and 13-course baroque lute music in the accord nouveau, transcribe the tablature as for a lute in E minor, not literally in D minor. This is not, in fact, a transposition, as there was no standard tuning of the lute or standard pitch in the baroque. (The pitches of surviving baroque instruments – woodwinds, organs, etc. – have been measured and found to vary not just microtonally, or about a semitone, but as much as a fifth.) The standardization of “early pitches” as used by the “Early Music” movement is a modern(ist) convention. The notion of playing “in the original key” was not a concern for actual early musicians. Transposition was a normal part of their performance practice (just note Bach’s approach to transcription, in which the key was almost always adapted to the nature of the target instrument; e.g., he normally transposed down a major second when transcribing violin concertos for harpsichord solo).
This is a general guideline, useful in most cases. For some transcriptions different solutions will have to be found.