Hey there DVenetian and Equator, I DO see what you are saying -thanks for your erudition on the subject of Perfect intervals.
Is that why the fifth note of a scale is called the dominant?
I finally see now why in a pair of notes how you decide which is the Tonic by counting one way and the other until you find which will be the fifth or not.
That is way cool.
Thanks again.

NOW - how does it work when you have say 6th String fret 3, 5th string fret2. This pair of notes still sound OK together, and it is the first and third of G major. But why is it G and not a chord (or DIAD) of B, which is the other note???

I do wish they had taught music theory at my school all those years ago.
I find it HUGELY frustrating not to understand the nuts and bolts of music, and now I have, maybe unlocked a little bit of the puzzle thanks to you guys.

Geoff Walker