I've become confused about what actually an auxiliary chord is. Is it a chord which doesn't belong to the key the piece is written in?
Is a chromatic chord an auxiliary chord which is achieved by moving one of the notes of the previous chord chromatically?
And are all auxiliary chords also chromatic chords?
Or are these two completely different things?
Also, non diatonic chords - are these also auxiliary chords?
An auxiliary chord is a chord formed by the presence of one or more auxiliary notes. An auxiliary note is a note that is not part of the primary harmony, but which connects notes within the primary harmony. An auxiliary chord can be diatonic (see example below).
An auxiliary chord, is an extension of the auxiliary note such that the duration of the auxiliary note and the way the note sounds in conjunction with the other notes of the chord, creates a sense of a change in harmony. (SOURCE)
For example, consider two root-position C major chords in which the G of the first chord moves to A, then back to G in the second chord. The A is an auxiliary note, and the A minor chord formed is an auxiliary chord.
A chromatic chord is any chord containing a note not native to the key.
A chromatic chord is a musical chord that includes at least one note not belonging in the diatonic scale associated with the prevailing key (SOURCE: Wikipedia, Chromatic chord)
A chromatic chord would be an auxiliary chord only in the presence of an auxiliary note.
Auxiliary chords don't exist as such. There are auxiliary notes, but they don't constitute chords. Auxiliary notes are non-chord notes, in between chord tones - as in chord C major - CEG, the aux. notes could be D, F, A, for example. They can be higher or lower, called as such. Generally diatonic in pitch.
Unlike chromatic chords (or chromatic notes for that matter), which are those not belonging to the key. So any chord which contains a chromatic (non-diatonic) note could be a chromatic chord, Like D major triad in key C, with its F♯.
