The tunes is an Am/D repeating using a A minor scale (C major) for leads. Where do you get a key of G??[/quote]
Key of G simply denotes the specific notes available, NOT TONAL ORIENTATION.
"In the Key of" can be interpreted in a couple of ways.
It can mean "We are using the G major scale and/or its relative modes" OR
"The key signature of this song is F# (because the G major scale and all of its modes use the same set of notes G A B C D E and F#)"
The easiest way to make sense of key is to understand it as implying a key signature, or a nonspecific group of notes.
So we have these ideas, seemingly intertwined: Key, scale, mode.
Well we've talked about what key means, what about scale?
Well a scale is a pattern of intervals that repeats every octave. As guiarists I hope we all know ATLEAST the intervals of a half-step (semitone, or one fret) and a full-step(tone, or two frets)
Heres the G major scale with the distance between each note.
G w A w B h C w D w E w F# h G
w=whole step h=half step.
This pattern is the same with every major scale, that is the "structure" of the scale that makes it major.
Now lets look at a scale like this (i saw some confusion on this earlier)
G VIII or I again
all those numbers mean are "this is the first (I) note in the scale, this is the second (II) note in the scale and this is the third (III) note in the scale, etc etc etc.)
so what is a mode? a mode is just a specific application of a scale. We "modulate" the scale, changing the TONAL ORIENTATION to a different degree., while still preserving the KEY or SET OF NOTES to be the same.
comparable to playing a video game in "different modes" so you can play that scale in different modes. Still running on same system or game (Key, or set of notes), but changing the way certain things work to change the experience without changing the game.
Were keeping the same notes, but changing the center of gravity away from the I chord (such as the G in G major, to say the II chord in G major, being Am)
Take the notes I II III IV V VI and VII of the major scale and build a chord of of each one, using only the notes of the scale, we get:
in the key of G this is
G major I
A minor II
B minor III
C Major IV
D Major V
E minor VI
F diminished VII
So based on what we talked about, we can use a progression of A minor to D major (Am-D), being a II-V progression, in the key of G.
Now to solo over it, he wants to use the notes in the KEY of G, but oriented around the A note. The A note is the SECOND note in the G major scale, and the MODE for the SECOND note is DORIAN (the minor scale is actually a mode, and its built off of the VIth degree, so Am scale is actually in the key of C, not appropriate for this song). But in actuality there is only a one note difference between the Key of G and the Key of C, so you could use Am over it without ALOT of problems, but try dorian you will like the sound.
So he is not using Am scale, he is using A Dorian.
So to sum up we have a progression Am-D (II-V) in the key of G, being solo'd over with the A dorian scale (which is the second mode of the G major scale).
So that even though the two chords are Am and D, and the scale is a A (dorian) scale, they are all in the key of G, because they all share the same groups of notes, it is merely the ORIENTATION of the HOME BASE (tonality) that make it all sound so confusing
so to sum it all up (very basically lol)
key = group of notes
scale = order of notes, and distances between them
mode = preserving a key but changing the orientation
hope i answered your question, but im sure if i did i only gave you 50 more to ask