I thought I would start a section on analyses of well known classical guitar works. It is always good to begin simply and who better to begin with than Fernando Sor (1778-1839). Sor has always been considered one of the best guitar composers of this period. His writing is straightforward and very well represents the classical style of composition. Sor was meticulous in his writing and was more concerned with musical correctness than guitar playability. His work is a text book example of musical mechanics and structure and sounds great as well!
Sor wrote many pieces for his students. To begin with we will examine some of his famous studies for guitar taken from a set entitled Douze Etudes (op.6) that he wrote while in Paris. The first etude in this set corresponds to Estudio four in the Segovia collection of Sor studies. I am working from a facsimile edition reprinted from the original Meissonier Editions with text by Frederick Noad that I received from the Guitar Foundation of America. I have included a performance edition with fingering as well as a clean copy showing harmonic analysis using traditional Roman numeral designations as well as lead-sheet style chord indications. I thought this may be a more practical way of showing the harmony as well as a way to bridge the gap between the classically trained and the non classically trained guitarist.
The piece is in D major so we would expect most of the harmony to be derived from the D major scale. The triads would be as follows:
D (I) Em (ii) F#m (iii) G (IV) A (V) Bm (vi) C#dim (vii)
The V chord (A) usually contains the seventh. Other seventh chords occur less often.
There are several examples of secondary dominants (measures 6,7,14,15,21,28 and 39) as well as the most striking harmonic feature involving the use of what is called an augmented sixth chord in measure 23. These chords are always the most difficult to understand until you actually see how they are used in the context of real music.
The most important thing to notice is the voice-leading that occurs in the two outer voices. Note how the G in the soprano (top) voice in measure 22 moves up chromatically to G# in measure 23 and finally to A in measure 24. At the same time the bass (lower) voice moves chromatically downward from B in measure 22 to Bb in measure 23 and finally ends up on A in measure 24. The interval that forms in measure 23 between the two outer voices is an augmented sixth (Bb to G#). The resolution of this interval to an octave (A to A) is the essence of this chord. Usually this augmented sixth chord precedes the dominant as it does here and involves the flat sixth scale degree (Bb) in the bass and the sharp fourth scale degree (G#) in the soprano. This is a perfect example of common usage of this harmony.
One other note: In jazz we call this augmented sixth chord a “tritone substitution”. I have indicated the chord sound in parentheses as Bb7. The actual spelling of Bb7 would require changing the note G# to Ab (enharmonic). If it were a Bb7 chord the resolution would traditionally be by descending perfect fifth. In this case it resolves down by half-step as it would when we encounter tritone substitutions in our jazz charts. So even though it sounds exactly like a dominant seventh chord, the augmented sixth chord is spelled and resolved in a completely different way.