Writing Passing and Neighbor Tones

A passing tone fills in the gap between two chord tones a third apart, moving by step in one direction. A neighbor tone steps away from a chord tone and back to the same note, either above (upper neighbor) or below (lower neighbor). Both are embellishing tones: they decorate the harmony without belonging to it. That is exactly why they can cause trouble. Every voice still sounds against every other voice at every moment, so the interval formed by an embellishing tone is just as real as one formed by a chord tone. If that interval happens to be a perfect fifth or octave, and the same voices had a perfect fifth or octave on the previous beat, you have written parallel fifths or octaves, even though one of the notes involved was 'just' a passing tone.

To add a passing tone safely, check every pair of voices at the exact moment the passing tone sounds, not just the chord tones around it. Picture the soprano and alto sitting a fifth apart on beat one. If a passing tone in the tenor on beat two creates another fifth with either of those voices, that's a parallel fifth, full stop. The fix is usually to choose a different voice for the embellishment, change its direction, or accept a different interval instead.

This matters for Roman numeral analysis because those labels describe harmonic function, built only from actual chord members. A passing tone between I and I6 is not F, A, or C membership-wise at that instant of dissonance, so it never earns its own Roman numeral, even though your ear hears it clearly. This sets up the next idea: once you can place single embellishing tones correctly, you are ready to string several together into suspensions and larger melodic figures.