Stable and unstable….what the funk!!!
This session fried my brain a little. We spoke of stable and unstable tones which, looking back, I defiantly understand it more now then I did then. lets see if can explain… so, the stable tones is usually the 1,3,5,and octave of a scale, as they are the major triad of the scale, and what we always lean towards for vocal harmonies. The unstable stones 4th,6,7th are the unstable tones.
Interest note, the maoris of New Zealand, total bad arse singers, huge stereotype but it is common as its a huge part of their culture. they are pretty sick at singing these harmonies.

C major scale CDEFGABC example
Stable tones C E G Unstable tones D F A B
1+2+3+4+1+2+3+4+1+2+3+4+1+2+3+4+1+2+3+4+1+2+3+4+1+2+3+4+1+2+3+4+ CDFCABCC