Chromatic Passing Notes
Chromatic passing notes are passing notes which do not belong to the current key. They normally occur between two notes which are a tone apart, filling in the semitone step.
The steps F-G and G-A are tones, so we could add chromatic passing notes between these steps.
Rising chromatic passing notes are normally written as sharps, and falling ones are written as flats.
Occasionally, chromatic passing notes can be used between notes which are a third apart, for example here. The key is C major, but a chromatic F# has been used to add interest to the melody.
Chromatic passing notes are used in places in the music where the composer wants to increase the tension or drama.
Chromatic Auxiliary Notes
Chromatic auxiliary notes are non-chord notes which are approached and quit in the opposite direction by a semitone step, and which do not belong to the current key. They occur between two identical notes.
The G# and D# here are the chromatic auxiliary notes. The B is a diatonic auxiliary note, because it belongs to the current key of C major.
Every step of the scale which is a tone (whole step) can easily be decorated with a lower chromatic auxiliary note. This note functions as a temporary leading note, moving to its own tonic (G#-A, and D#-E in the previous example). They are notated as either sharps or naturals, depending on the key.
It is usually best to avoid a semitone clash between the 3rd and 4th degrees of the scale, as this can sound very harsh. Sometimes a lower chromatic auxiliary note can be an effective solution to this problem.
In the first example below the A (bass note) and Bb (lower diatonic auxiliary note) sound harsh. In the second example, the B natural sounds softer because it is a tone step away from A, rather than a semitone. (Another fix could be the third example, which removes the A from the bass completely.)
Upper chromatic auxiliary notes cannot easily be applied to every note of the scale in the way that lower chromatic auxiliaries can be used. They will often produce strange-sounding results, like this example:
This is because the upper auxiliary note doesn’t have a clear function in the way that the lower auxiliary note does (i.e. the lower chromatic auxiliary works like a leading note). The major and minor versions of a key can become blurred, and the result is harmonic confusion.