Site blog: Online Music Theory & History
Often in music we see notes that do not correspond to the written key signatures, notes with added sharps, flats, and naturals. There are many reason for these chromatically altered notes.
Sometimes a sharp or a natural sign is simply a raised leading note for a minor key. Perhaps it is a chromatic non-chord tone such as a passing tone connecting two notes a whole step apart. Sometimes an accidental indicates a secondary dominant, or even a change in key. These topics are covered in the R.C.M. Basic & intermediate Harmony program.
There are also a whole set of chromatically altered chords used to colour chord progressions, or to intensify certain harmonic progressions. Pop musicians are familiar with modal interchange, the borrowing of chords from one mode to another. In the classical world this technique is usually called modal mixture. For example, in C major, an F minor chord may be 'borrowed' from C minor, lending a minor colour to the subdominant chord.
There are also a group of chromatically altered chords, that for historical reasons have regional names: Neapolitan Sixth chord, German Sixth, French Sixth, and Italian Sixth. We study these chords in Advanced Harmony.
In the opening measures to Beethoven's Moonlight Sonata, we see the use of such a chord, the Neapolitan Sixth chord, usually identified in music analysis as simply N6. This chord is a major triad build on the flattened second scale degree. It is often found in first inversion.
Beethoven's Moonlight Sonata is in the key C-sharp minor, but in the third measure we see a D-natural with F sharps and an A. Together these pitches make an N6 chord: D F-sharp A, with F-sharp in the bass, the II chord flattened by a semitone in first inversion. This chord then moves to a dominant seventh chord in root position, mm. 4.
When we listen or play the music we can hear how this Neapolitan sixth chord helps create the character of the opening chord progression and the piece itself.