Yeah, sorry, let me backtrack a bit. This is all very heady stuff, so if you're basic understanding of theory isn't rock solid, then you might miss some of this.
All of contemporary analysis in jazz/rock/pop/whatever is based on the assumption that the major scale is the "default" key. The pattern of W W H W W W H in terms of half/steps and whole steps is the basis of most harmony and melody in traditional Western music, and so everything else in Western music from then on out was based upon comparison to the major scale. Formulae for deriving not only other scales, but chords, intervallic patterns, etc are based upon comparison to the "default" major scale, which is numerically represented as "1 2 3 4 5 6 7", with each number representing that particular degree of the major scale built on the first degree. If you know the notes of the major scale associated with a particular tonic note, then to find any other scale/chord/chord progression it becomes a matter of applying a formula. Formulae are typically represented by a similar numerical order and application of accidentals. These accidentals don't literally mean that the accidentals are applied to the notes on the staff as they appear, but rather that those notes of the major scale are adjusted up/down a half step (sharp meaning up, flat meaning down)
For example, lets try and spell Ab lydian. First, figure out the key signature for Ab (4 flats) and then apply to a generic "A" scale.
A B C D E F G
becomes
Ab Bb C Db Eb F G
Next, apply the formula for lydian, which is 1 2 3 #4 5 6 7. This is essentially the same as the major scale, but the fourth degree has been raised (or "sharped") In this case, you're not going to get a D#, however, but a D natural since the D was flat already and raising it yields a natural D.
Ab Bb C D Eb F G
If all of these seems trivial to you, thats fine, but its to prove a point that everything in music is described in terms of the major scale. Spelling a G-7(b5,b13) chord yields 1 b3 b5 b7 b13, or G Bb Db F Eb, which are all their corresponding changed notes from the G major scale. It may be confusing to be in the minor key, but still think of the notes from the major scale, but thats how its typically done, and for the vast majority of instances, it makes things a lot easier the more in-depth you do in harmony.
