What Scale?

You're sort of right.

As you change keys around the circle of fifths, each successive key changes only one note. If you're starting in C, and you then sharp all the F notes, you get the scale fingerings of the key of G (the V chord). If you're starting in C, and you then flat all the B notes, you get the scale fingerings of the key of F (the IV chord). This more or less explains why the I-IV-V chords work together so well.

As you suppose, you could use the fingering from a different key. If you use the A minor pentatonic scale fingerings over an E progression, you're playing the notes A-C-D-E-G; the E minor pentatonic has E-G-A-B-D. You've essentially traded the B note for a C note. The C is the b6 of E - it'll sound better over an E minor progression than it will an E major progression (because C is in the E minor scale, but not E major). You'll be giving up B, which is the fifth... which is a pretty useful tone for ending your intermediate phrases.

A better choice might be the B minor pentatonic. That's got the notes B-D-E-F#-A. Here you keep both the root and fifth (the two most "solid" tones) and trade in the G note for F#. That's the sixth note of the E major scale. In fact, the only note that's not in the E major scale is D (which is the b7). So you'll be edging towards a mixolydian sound - the E mixolydian scale would also have the notes G# and C#.

But as far as scale fingerings belonging to some family group, that's a dead end. You can think of scales as being grouped by similar features - for example, ALL minor scales have a b3 in them - but the relationship is really between the pitches used and the tonic. If you're going to go to the effort to figure out those relationships by fingerings, your time would be better invested if you learn the actual note names on the fretboard and the spellings of the scales - that knowledge will have a lot more use, and give you more direct access to the theory underlying music in general.

Thanks Noteboat,and just when i think i am getting somewhere lol.Aw well,back to the beginning again with another scale.

A and E are of course closely related. E is the 5th of any A scale and belongs to both A minor (A_BC_D_EF_G_A) and major (A_B_C#D_E_F#G#A) and of course to both the A pentatonic minor (A__C_D_E_G) and pentatonic major (A_B_C#__E_F#__A)

Here's a table of all the keys and their relative minors that may help you see the relationship between them:
Major Relative Key I ii iii IV V vi vii°
Key Minor Signature 1 2 3 4 5 6 7
----------------------------------------------------------------
C major A minor C D E F G A B
G major E minor # G A B C D E F#
D major B minor ## D E F# G A B C#
A major F# minor ### A B C# D E F# G#
E major C# minor #### E F# G# A B C# D#
B major G# minor ##### B C# D# E F# G# A#
F# major D# minor ###### F# G# A# B C# D# E#
F major D minor b F G A Bb C D E
Bb major G minor bb Bb C D Eb F G A
Eb major C minor bbb Eb F G Ab Bb C D
Ab major F minor bbbb Ab Bb C Db Eb F G
Db major Bb minor bbbbb Db Eb F Gb Ab Bb C
Gb major Eb minor bbbbbb Gb Ab Bb Cb Db Eb F
----------------------------------------------------------------

Note that there are 13, not 12, because F# and Gb contain exactly the same notes, but with different names.

There are actually 15 - B (5 sharps) is also Cb (7 flats) and Db (5 flats) is also C# (7 sharps)

Using 7 accidentals is rare, because it's generally easier to read in the alternative. But they DO occur in actual music when it makes sense. The last time I saw seven accidentals was probably six or seven years ago - it was an ABA piece in a minor key with the B section in the tonic major.

Because the A section was in C#m (relative to E major, 4 sharps), the composer put the B section in C# major. In context, that made a lot of sense, for several reasons:

1. It means you're changing the way you think about note names in standard notation for just three pitches (A->A#, E->E#, F->F#) instead of all seven (A->Ab, B->Bb, C#->C, D#->Db, E->Eb, F#->F, G#->Gb).

2.If you had tried to keep the whole thing in E and handle it with accidentals you would have needed a lot - because the B section was in a major key, you'd be using a sharp for the first A, B, or E in every measure. That would have required more than 60 accidentals in the B section, compared to a one-time signature change at the start.

3. It prevents you from having to shift your thinking about the key note twice in the piece, and having to adjust your reading from a sharp key to a flat one.

So in that case, it was actually easier to read in seven sharps than it would have been in five flats. But when I say it's rare, I mean it's RARE! In the 40 odd years I've been playing, I've played thousands and thousands of pieces of music. I've seen maybe five that used seven accidentals, and none that used seven for an entire piece.

Thanks to you all for replying and helping me.So the moral of the story is yes you can if it sounds ok but be careful lol.