Originally Posted by Dimebag
I thought that flats and sharps can't be in the same scale.
All sharps or flats can be labelled either sharp or flat.
The only reason we choose one or the other is to ensure that all notes are used in the scale, and also it is better not to mix symbols.
Take the D major scale for example:
It has 2 sharps: F# and C#
D E F# G A B C#
It could also be labelled with flats:
D E Gb G A B Db
As you can see, the are 2 different D notes (Db and D) and 2 different G notes (Gb and G). Also, the notes F and C do not appear.
This is how we decide whether to use flat or sharp symbols, to ensure all 7 notes (ABCDEFG) get a mention in the scale whether # or b.
On most scales, this will mean that there will be either sharps or flats, but not both.
That scale is so unusual that it is impossible to observe the basic rule of including all 7 note names and not observe the lesser rule of not mixing sharps or flats.
By the way, intervals are as follows:
C D Eb F# G Ab B C
W H W+H H H W+H H
W=whole step (2 frets)
H=half step (1 fret)
W+H = whole + half step (3 frets)