Narciso Yepes invented the 10-stringed guitar. What are its advantages? Are there any schools where playing it is taught?
I own a 10-string classical guitar.
The two most common ways to string and tune it are the Yepes tuning, and "Baroque". Mine is strung in a modified Baroque fashion. [Many professional 10-string guitarists have developed their own alternate tunings, particularly when they want to compose and play their own new music and not transcriptions of historical music.]
In the "Yepes" tuning, you only play the top 6 strings; the other 4 strings are there to provide sympathetic vibrations. The regular 6 strings are the same as usual; the lower 4 are C (.050), Bb (.028), Ab (.0345), Gb (.042). Notice that the Bb, Ab and Gb are higher in pitch than the regular 6-string guitar's low E string.
In the "Baroque" tuning, you play all ten strings, although the bottom 4 are generally only used as open strings (sometimes called "harp" strings because you just pluck them like a harp; you don't fret them). The lower strings are A (.056), B (.054), C (.052) and D (.050) below the pitch of the regular 6-string guitar's low E string.
With the Ypes tuning, you get much more sympathetic vibration and resonance from all the notes you play on the top 6 strings. This is called the "cathedral guitar" effect because the guitar sounds like it is producing its own reverberation. It sounds a bit like playing a piano with the damper pedal down.
With the Baroque tuning, you can play music written for the lute and make use of the extra low bass notes that were found in the original lute arrangements. (when lute music is transcribed and arranged for the guitar, a lot of low notes have to be left out.)
The most affordable 10-string guitars on the market are made by a Spanish company called Bartolex. The guitars, however, are built in China. Bartolex frequently sells guitars directly on eBay, but there is also a shop in the USA, Cathedral Guitar, that sells them. Bartolex has a shop in Spain; check their Web site.
There are several luthiers around the world that hand-build professional quality 10-string guitars, but that is beyond my budget.
The guitar I bought is the simplest and least-expensive Bartolex model. I'm not an accomplished guitarist. To be honest, I bought this guitar merely because I was curious about it. I don't play it nearly as much as it deserves to be played.
Thoughts on playing one
It's a lot easier to get used to than you think. The instrument is neck-heavy and the physical balance takes some getting used to. However, you can play it exactly like you play a 6-string guitar; the lower strings only come into play when you need them. If you are playing Baroque-style, you have to learn to use your thumb across a wider range of space to pluck the lower notes. The low strings are expensive and have to be special-ordered, but you don't have to replace them often since you don't pluck them very much and you don't fret them at all. So you can put the lower strings on and leave them there while you replace the upper 6 strings as often as you usually do.
Just Google "10-string guitar". There are many recordings for you to listen to, and videos on YouTube.
Here is a good introductory video from professional guitarist Perf de Castro.
There is an International 10-String Guitar Society run by guitarist Janet Marlow. They have annual conventions for players and they have a lot of resources on their Web site.
Some information gleaned from Wikipedia and elsewhere:
Narisco Yepes helped José Ramírez design a 10 string guitar in the early 1960s, but Yepes was keen to point out that he "invented nothing", since guitars and lutes have had various string configurations over the years.
The intention was that 6 strings would be tuned conventionally, and played as normal. The remaining strings vibrate sympathetically. The standard tuning is chosen so that there is a sympathetic vibration for every note in every scale.
Soon, composers began writing specifically for the instrument. The earliest listed in my sources is Tiento by Maurice Ohana, originally written for 6 strings, but adapted for 10 strings in the early 1960s.
tenstringguitar.info keeps a list of compositions here. That site is a good source of general information on the instrument.
Three points often claimed that are actually false:
1) The claim that the Yepes (Standard) tuning is used ONLY for resonance. Absolutely false. Merely by watching the videos of Yepes on youtube (eg. the Kucera, Brouwer or Kuehnel pieces) or by studying compositions written for Yepes's 10-string guitar (eg. Ohana's Si le jour parait) or by reading his interviews, it can be seen unequivocally that all strings are plucked by the right hand and even fretted. Professional artists use the instrument the way Yepes designed it, with its standard tuning; that is, use it FULLY, for resonance AND for plucked AND for fretted basses.
2) The claim that a "baroque" tuning is what is required for Baroque lute music. This misinformation stems from the erroneous terminology used years ago in the LaBella strings' catalogue, which has since been corrected to "Romantic" tuning as this is the tuning of a 19th century harp guitar played by J.K Mertz and, considered in its totality, in no way a relative tuning of any kind of baroque lute. This so-called "baroque" tuning was never used by Narciso Yepes, who even said so in an interview published in Soundboard. I can confidently say that this "baroque" tuning does NOT lend itself to an informed performance of baroque lute music, which must be transcribed in E minor for the guitar, not D minor, to transcribe not only the skeletal notes, but also the implied and stylistically compulsory ornaments that revolve around the use of open treble strings, of which the guitar has E, B and G (thus E minor) not D, A and F (or D minor) like the lute. This means, in E minor rather than D minor, that a low A bass is redundant for 13-course lute music. Moreover, music written for 11-course baroque lute requires G(#) and F(#) as well as E, and low D or low C basses, exactly those which are available on the 10-string guitar as Yepes designed it. The low B and A strings of the so-called "baroque" tuning once again are completely redundant and intended for 19th century harp guitar music, not for the vast majority of lute music.
3) The innovation Yepes made is not termed "cathedral" effect, as some call it. This is a plug for a brand name of budget instruments (Cathedral Guitars) that, being strung and tuned in a way that (as a fact of the science of acoustics) denatures the salient characteristics of Yepes's innovation, have NOTHING to do with Yepes's important innovation.
For an answer to the posed question that comes directly from Narciso Yepes, one simply has to visit my website www.tenstringguitar.INFO where the verifiable journal articles quoting Yepes give the following:
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.)
In the first place, the four supplementary strings [C2, Bb2, Ab2, Gb2] 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, A, B 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. This allows me to modify at will not only the volume but also the tone-colours.
(Narciso Yepes. The Ten-String Guitar. Trans. Lionel Salter. La Cantarela, July 1973.)
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 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.)
To answer the second question: There are schools where the 10-string guitar is taught in a way that is aligned with Narciso Yepes's true conception of it. In South Africa, Fritz Buss (a top direct disciple of Yepes) and his top student Viktor van Niekerk, in America Jonathan Leathwood (not directly of the Yepes school, but a respected scholar and player who uses the instrument as Yepes designed it), in Switzerland Stephan Schmidt (same as Leathwood); in Spain Ismael Barambio (a top direct disciple of Yepes), in Belgium Godelieve Monden (a top direct disciple of Yepes), in Germany Andreas Hiller (a top student of Stephan Schmidt), and in Japan various former students of Yepes.
(I have direct access to Narciso Yepes's unpublished, handwritten transcriptions as well as his student whom he publicly and verifiably touted as one of the best teachers in the world, and I am a professional 10-string guitarist and teacher who has read for a Master's in performance on the 10-string guitar specializing in transcription of baroque lute music.)