Section 28.2 Avoiding Objectionable Parallels
When adding non-chord tones, one must avoid creating objectionable parallels. To avoid creating parallel 5ths and 8ves when adding non-chord tones, first identify intervals of 5ths and 8ves between voices in your original voice leading.
Avoid adding non-chord tones that move by step into notes separated by a 5th or 8ve.
It is best to avoid adding non-chord tones in two voices on the same beat unless they are moving in parallel 3rds or 6ths.