In order to beef up the repertoire, guitarists have long looked to music written for other instruments. An early example is Luys de Narvaez' transcription of a chanson by Josquin des Prez for vihuela, a predecessor to the guitar. Robert de Visée transcribed an overture by Jean-Baptiste Lully for both baroque guitar and lute. Here is a performance:
Tarrega transcribed music by Chopin for guitar. In the 20th century the practice of transcription took off in a big way and guitarists looted the repertoire for lute, harpsichord and piano to adapt to their instrument. They were so successful that some pieces originally for piano, like "Asturias" by Albéniz, are now better-known on guitar. Segovia was a particularly successful transcriber and it is his version of "Asturias" that is most often played today. Another very successful transcription was his adaptation of the famous Chaconne from the D minor violin partita. He transcribed a lot of other music by Bach as well. The most promising source of repertoire suitable for the guitar turned out to be the music for solo violin and cello. Bach also wrote music for lute (though it was likely composed not for an actual lute, but rather for the keyboard instrument that sounded rather like a lute called the "lautenwerk"), though the textures are sometimes too thick to be comfortable on guitar.
One of the most successful transcriptions, made by John Duarte rather than Segovia, was of the first cello suite, originally in G major, but transcribed in D major. I started out to work on this with José Tomas when I studied with him in Spain. He had some misgivings about the Duarte transcription. The problem is that when you put the whole suite up a perfect fifth, from G to D, it sounds rather thin and in need of a bass line. So Duarte supplied one. The only problem with that is that neither Duarte nor anyone else is really capable of writing a bass line that would do justice to a piece by Bach. Tomas, who played an eight-string guitar going down to a low C, played through part of the suite in the original key and convinced me that it had much more sonority down low. When I got time, I sat down and figured out that on a six-string guitar the best option was to go as low as possible. With the sixth string tuned to D, that meant A major. So I transcribed the whole suite in that key. Apart from a couple of notes suggested by Oscar Ghiglia in the gigue, I didn't feel the need to add anything!
UPDATE: I forgot to mention that Pepe Romero followed exactly the same procedure in his transcription of the Third Cello Suite which he plays in D major, up a tone from the original C major--and with virtually no added notes.
It is an interesting question why transcribing Bach is often successful, but we avoid transcribing Beethoven, Mozart and all those other great composers. I think it is because Bach's music is not so tied to whatever instrument he originally wrote for. If you can play the notes, then the piece is going to come across. But we shudder to think how playing Beethoven's Moonlight Sonata on any other instrument would distort the effect.
I will put up the whole suite, which is in six movements, but for today, here is the prelude:
This suite is included in my book of Bach transcriptions, now out of print. I am thinking about re-issuing it as an e-book through Amazon. Here is the link to the original edition.