For a non-chord tone to be a non-chord tone, it has to occur during a given chord where the tone is not part of the chord.
There are three parts: Preparation (before non chord tone), the non-chord tone itself, and the resolution (note after the non-chord tone).
Non-chord tones can be:
Passing Tone (PT) Is approached by step and left by step in the same direction. Ex: Both preparation and resolution are moving down(or the opposite direction).
Neighbor Tone (NT) Is approached by step and left by step in the opposite direction. Ex: The preparation moves down and the resolution returns back up(or the opposite for each). Always end on the same note they start on. (At least for this class).
Suspension (SUS) Is approached by static motion and resolves downward by step. All suspensions require two chords as the preparation will be on a different chord than the NCT itself. You have to have this downward motion. If you don’t, you may have to alter the chord when inserting a suspension. Suspensions will always be on the chord change. We label how suspensions resolve with its intervalic label. Anything above the bass voice, you write the interval of the two notes of the suspension against the bass note.Ex: C in the bass with a D to C resolution in the alto voice is a “Sus 9-8.” If this example occurs in the tenor voice, it is considered 2-1, in which the tenor and bass voices resolve to the same exact pitch. The most common suspensions are 4-3, 7-6, 9-8.
If the suspension is in the bass, you measure it against the most dissonant voice. They will always be labeled 2-3 because there will always be a second. Also note that the numbers get bigger when labeling. (4-3 vs 2-3)
Retardation (RET) Is approached by static motion and resolves upward by step. You can think of it as a suspension that resolves up. It follows all of the same rules. Retardations do not need numbered labels, such as with suspensions. These are much less common.