Hi guys 'n girls!
This is probably a question/statement for Jerome about flats/sharps... (don't know if I dare get in to a discussion on theory with the Jerome guru :-)
In my (swedish) book basic theory for guitar the explanation for why you choose to say flat or sharp (ex. F sharp or G flat) is that in a scale there can never be both a "full" note and a sharp/flat with the same name.
If for example the scale contains a standard F, there can't be a F# (or a Fb) in the same scale, it will then be called Gb (or E#) instead (same shit, different name :-).
And now some theory to enlighten you (you already know this Jerome):
If you build a major scale it's composed by stepping through the scale (the full scale is ALL notes A, A#/Bb, B, C, C#/Db, D, D#/Eb, E, F, F#/Gb, G, G#/Ab) by this order:
Root, full step, full step, half step, full, full, full, half (and you are back to the root 1 octave higher)
If you use that and create for example a D major scale you get the notes:
D (root), E, F#, G, A, B, C#, D (back to root). To see this theory, look at the D string on your guitar, start with an open string (note D) and walk up the neck on that string by the steps above.
In the D major scale you have "jumped" the F and C notes and got F# and C#. There is already a G and a D in the scale so you can't use Gb and Db.
Now build a F major scale the same way and you get:
F (root), G, A, Bb, C, D, E, F
The first half note step goes from A (which you therefore already used) and you get a Bb, of course it's the same as A# BUT you can't use both A and A# in the same scale which instead in the F major scale give you a Bb.
Mostly as a hobby guitarist you don't now (or don't care) what scale the songs are built of, but the creator probably knew the theory and give your brain some problems by using F# in one song and Gb in the next...
If you understood anything of this jidderich you can just as easily build a minor scale. Just learn the note steps: full, half, full, full, half, full, full and use it from your root (the root gives the scale it's name)
Don't now if this confused or explained...
/Missen