Originally Posted by TangledMortalCoil
yes, i meant shifting up one fret towards the bridge. thanks for your replies... to make a long story short, i needed to know because i had this riff i made up and i was trying to turn it into a 2-guitar harmony, so i first had to figure out what scale/mode it fit in.. and the phrygian shape worked.
let's say that i had a completely improvised riff and i wanted to harmonize it, but it didn't happen to fit into any ascertainable mode.. is there any other way? i mean how could you calculate, say, 3rds if you didn't know what scale/mode you were in??
It's highly unlikely you'll invent a riff which doesn't fit some scale or other. But assuming you can't identify it...
Apart from davie's suggestion, you could pick any other note from your scale.
I mean, I assume your riff will contain at least, say 3 or 4 different notes. Treat those notes as the scale, whether or not it matches any scale you know. Just use other notes from that set as harmonies.
If in doubt - or if the intervals don't seem to work - use your ear: pick any note, and move it this way or that until it sounds right. (You'll probably find that this exploration adds missing notes from the unknown scale, helping you identify it.)
Your ear is always the best method when harmonising. Even if you know the scale you're using, don't feel you have to stick to it when harmonising. Normally you would (that's safe and easy, and will work), but occasionally chromatic notes work better.
Using your ear can be slow - it may take several attempts (including recording each harmonisation and listening to it) before you're happy with the result.
It can also help to play the harmony alone (without the lead line), to see how it sounds. Good harmonies make their own melodies or riffs.
Even so, quite often, you end up with choices you can't decide between - which is where theory can help.