Two main versions of ten-string guitars exist (both are classical guitars) - a romantic ten-string guitar, with four or five "floating basses" that are not fretted, and a modern ten-string guitar (with frets under every string), which was originally designed with a singular tuning in mind, with the aim of enhancing and balancing resonance.
Different romantic ten-string guitars exist
Johann Kaspar Mertz is known to have played ten-stringed guitars. Based on surviving instruments and urtexts of music written for it, the following tuning was used:
Versions of the Décacorde were made by René Lacote and two of his Décacordes are housed in the Music Museum of the Cité de la Musique in Paris:
Neither the Décacorde nor the versions by Scherzer can be said to have flourished, but the Decacorde had little repertoire except for a Method book and some Divertissements by Carulli.
Narciso Yepes (1981): "My reasons [for inventing the modern 10-string guitar] were purely musical, and the first of them was that the guitar was not properly balanced. There was no equilibrium, because of the 12 notes of the scale, only four - E, A, B, D - had any resonance. If you play one of those notes and then stop the string with your finger, you will hear the sound lingering. But if you play one of the other eight notes of the scale, the sound dies immediately. On the 10-string guitar, I have resonance on all 12 notes."
After Yepes had already achieved international fame, he reached the point where the 6-string guitar no longer sufficed for his needs. He was disturbed by the irregularity of resonance produced by the overtones of its bass strings, vibrating in sympathy with notes played on the fingerboard. Four notes in particular (E, A, D, B) sounded full, enriched by this sympathetic vibration, while the other eight tones of the chromatic scale were without the same lustre and sustain. Yepes's idea to correct this imbalance - a guitar with fully chromatic string resonators created in 1963 in collaboration with José Ramirez - followed a strict musical and scientific logic.
Upon adding four bass strings tuned a very specific way - C, B-flat, A-flat, G-flat - the same sympathetic resonance is elicited by each of the notes that make up the fingerboard's sonorous catalogue, by taking advantage of the natural harmonics (the octaves as well as the fifths) of the bass strings, which produce unison, sympathetic vibrations with notes played on the fingerboard. In other words, the additional strings act as tuned resonators, or string resonators, that sustain and enrich the sound. (That is not to say that these strings are not played. They are indeed fingered with the left hand and/or sounded by the right, if/when this is required by the musical context). Thus, the C-string adds the resonance for itself, its octaves and their fifths, i.e. G's; B-flat resonates with B-flats and F's; A-flat resonates with A-flats and E-flats; and G-flat with G-flats and D-flats, completing the string resonance for the twelve tones of the chromatic octave.
Yepes: "This does not mean a break from nor lack of respect for the admirable instrument of tradition. My new guitar is not basically different in sound colour, timbre, nor technical approach from the 6-string guitar. Imagine a piano without a pedal which suddenly acquired one - what new possibilities in the enrichment of sound this means is self-evident."
This result could be termed linearised chromatic resonance since the bass strings now resonate equally in sympathy with any of the twelve notes of the chromatic scale, similar to the piano's sustain when the pedal is used. And just as a pianist has the option of whether or not to employ the pedal, the competent 10-string guitarist is able to execute complete control, sustaining or stopping notes at will. "If I have resonance, I can stop it. But first I must have it. You see, the problem is not in the guitar, but in the player." (Yepes 1981) With respect to the traditional 6-stringed guitar, this chromatic resonance and equal timbre of tone are intrinsically absent from it, but also equally unachievable with any tuning of the 10 strings other than the one invented by Yepes.
Yepes (1981): "But that is not my only reason. If the guitar is to the lute what the piano is to the harpsichord - that is, a new expression of an old instrument - then, I should be able to take a piece of music composed for the lute and play it directly on the guitar, without making any [alteration] in the text, just as a pianist can play a harpsichord work of Bach or Scarlatti. This cannot be done on the six-string guitar, because the lute had more than six strings, especially during the Baroque period. At the same time, having the expanded range of the 10-string guitar makes it possible for me to approach [more faithfully] the music of Albeniz, Falla and other Spanish composers inspired by the guitar, but who composed for the piano. I can play their music as it is written, with no sacrifices."
Thus, it now becomes possible for the guitarist to play Bach and repertoire written for the Baroque lute without deleterious transposition of the bass notes, by employing scordatura, lowering the tuning of the 7th string (the one with the lowest pitch) to BI or AI - in Scientific pitch notation, B1 or A1. A ubiquitous misconception (also among 10-string guitarists) is that these additional strings are intended to simplify the execution of bass notes by playing as many of them as possible on open strings, as on the lute. However, this approach is not consistent with the performance practice of Narciso Yepes. Actually, he played all notes between the tones of the open 6th and 7th strings as stops on the 7th string, not on individual open strings. This is evidenced in autograph manuscript sources indicating his own fingerings, which also show an implementation of the open (and stopped) strings 8-10 that, however, never becomes gratuitous or perfunctory.
Aside from the fact that the instrument opens up possibilities for more faithful transcription of music originally written for lute or keyboard, it also opens up new possibilities for original composition, as exemplified in the solo guitar works of the great Modernist composers Maurice Ohana and Bruno Maderna who wrote substantial works specifically for this instrument.
Yepes (1981): "Of course, the final reason is that, if I have a 10-string guitar, I have within it a six-string guitar; but if I have only six strings, I do not have 10. I have all the advantages and none of the disadvantages." However, he warns, "it is very difficult to find a well-made 10-string guitar, and the number of poorly made ones on the market have led many guitarists to assume that those instruments are bad because they have 10 strings. No - they are bad because they are bad!"
(as written in the Helmholtz pitch notation system) which can also be written enharmonically as:
In the so-called Scientific pitch notation, or American system, the tuning is written (from 1-10) as:
This is the string configuration and tuning, that was originally developed with the idea of enhancing and balancing sonority, by allowing bass strings to vibrate in resonance with all 12 chromatic notes.
NB. In both pitch referencing systems the octave starts on C. Thus, correctly, string 7 or C should be that with the widest diameter and lowest pitch. String 8 is thus a minor seventh above string 7, not a whole tone below it. Numerous authors, apparently not au fait with these systems of notation, have misrepresented the instrument's tuning in print. It is thus no surprise that numerous guitarists have also adopted erroneous string configurations in order to tune one or all of strings 8, 9 and 10 an octave lower than they ought to be.
Prior to 1963, a number of different types of guitars with ten strings were played by, among others, Johann Kaspar Mertz and Ferdinando Carulli. The first played an instrument with four additional free-floating basses tuned diatonically from D to AI. Carulli called his instrument the Decacorde, which was tuned eI-b-g-d-A-G-F-E-D-C. (The last five strings are not fretted.) Indeed, if one is to do justice to the music of numerous 19th century composers who wrote for instruments with more than six strings, these period instruments would be most suitable.
Taking nothing away from their suitability for the performance of 19th century repertoire, it has to be pointed out, however, that the concepts behind these guitars are contrary to that of the Yepes ten-string guitar, since the tunings of these instruments were not intended to resolve - and do not resolve - the problems of resonance inherent in the design of the guitar. As Yepes said about instruments that add redundant resonances such as B, A, or D: "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" (Yepes 1978: 46).
However, to complicate matters, since 1963 ten-string guitars that seem to be modern in appearance have been appropriated by some proponents of the abovementioned Romantic ten-stringed guitar, tuning the additional strings diatonically from D to AI (a system also known by the misnomer "Baroque" tuning). This has led to some confusion between two visually similar but conceptually disparate instruments: on the one hand, the Romantic ten-stringed guitar (whose purpose is an extended bass register - one that, inadvertently, augments the guitar's resonant imbalance), and, on the other hand, the Yepes ten-string guitar, whose raison d'être is, first, linearised resonance for the entire chromatic octave, and second, an extended bass register.
One cannot consider as synonymous (just because they have the same number of strings) different instruments that do not have a commonly accessible original repertoire, that approach music through different performance practices (different techniques, especially with respect to the use of the 7th string, open and stopped strings), different instruments that are not only tuned differently but strung differently, and most problematic of all, one cannot simply disregard that the true modern 10-string guitar [for lack of a better term than '10-string'] was invented, by definition, first and foremost, to linearize (to balance, to create consistency among) all twelve tones, by means of a singular tuning based in an understanding of physics, so that any note could be sustained or stopped, brought out or underplayed as the Musical context requires, not as instrumental limitations necessitate. As such, the guitar comes to possess only those qualities that have already been common but vital features of (to a certain extent) the harpsichord and (especially) the piano, instruments that have set the standard of the concert hall for centuries. The true modern 10-string guitar is as little defined by its number of strings for their own sake, divorced from their singular tuning, as a piano is defined by its number of keys. The modern 10-string guitar proper is defined by the introduction of additional strings (resonators) possessing only those overtone frequencies corresponding to resonances that are missing or weak on the traditional guitar (by which is meant the overtone frequencies corresponding to the octave and the perfect fifth above the open string, and to a weaker extent, the double-octave and its perfect fifth), and introducing no redundant resonance of D, A, E, B, which already receive this sympathetic support from the basses of the (6-string) guitar. In addition, it features an extended bass range that opens the baroque lute repertoire to more stylistically faithful transcription on the guitar. However, on the modern 10-string guitar proper, a number of these basses are stopped [i.e. 'fretted'] on the 7th string in keeping with the performance practice of classical guitarists (as opposed to lutenists). If the point were to play basses on open strings, an instrument with at least eight diatonic basses below the 5th string would be required to do justice to baroque lute music. Attempts to import a lutenist's performance practice, such as those witnessed in transcriptions of Bach and Weiss for the 'Marlow Method', introduce stylistically impermissable augmented octaves and other compound intervals and leaps where there should be movement in 2nds, since (in contrast to the lute) the instrument does not possess a true diatonic scale in the open basses (lacking the required number of strings). On the modern 10-string guitar proper, the bass notes can be stopped/fretted on the lowest string, seven, at the correct pitches, maintaining the correct melodic intervals.
As such, there are period instruments of the 19th century with ten strings, which are perfectly suited to the performance of repertoire written specifically for such instruments; there is the true modern 10-string guitar invented in 1963, with its singular tuning; and then there are guitars that happen to have ten strings, which can be tuned any way their players like. However, they are three distinctly different types of guitar that have little more in common than the number of strings.
Baroque guitars typically had five courses, of which either four or five were double-strung making a total of nine or ten strings. (A course is a pair or more of strings that effectively function as one string.) While many baroque guitars have ten strings, they are not normally called ten-string guitars. Note that this terminology clashes with that of the modern twelve-string guitar, which has six courses each of two strings.
Yepes, Narciso. 1978. "The 10-String Guitar: Overcoming the Limitations of Six Strings". Interview by Larry Snitzler. Guitar Player 12(3): pp. 26, 42, 46, 48, 52.
Yepes, Narciso. 1981 "Narciso Yepes and His 10-String Guitar" Interview-Article by Allan Kozinn. The New York Times, Nov. 22: p. D21