bb7 and b6

Just fyi a bb7 is enharmonic to a 6 not a b6.

But to answer your question the only way to tell is context. I think this will explain.

Take the major scale 1 2 3 4 5 6 7 1. In that spot between the major 2nd and the major 3rd there is a note that could either be a b3 or a #2.

If you altered the scale by flatting the 3rd making the new scale 1 2 b3 4 5 6 7 1. Then it would be a flat 3rd.

Alternatively you could alter the scale by raising the 2nd, this would make your new scale 1 #2 3 4 5 6 7 1. Now although that particular note the #2/b3 is in the same place look at the 3rd, it is still major right where you left it. In this context you would call that note a #2 and not a b3.

Not only would you call it something different, it SOUNDS different.

Your example in particular of the bb7 would be correct if your scale was say, 1 2 3 4 5 b6 bb7. Notice that the 6th must be flat in order to make room for the double flat.

If your scale was 1 2 3 4 5 6 b7 you would call that same note a 6.

so why wouldnt it be:

1 2 3 4 5 b6 6

instead of

1 2 3 4 5 6 bb7

?

Because you're really only supposed to use each numbered scale degree, or note letter, once.

Plus, those are still not the same since one uses b6 and the other just has 6.

you couldn't have a 6 and a bb7 because they are the same.

And the whole point of using these numbers is that you do not repeat them. You move them like a slider. Seems like your not actually playing them on guitar and your just throwing numbers out there.

so why wouldnt it be:

1 2 3 4 5 b6 6

instead of

1 2 3 4 5 6 bb7

?

It depends on the context.

Scale numbers are a shorthand for the letter names of pitches, and when you have a scale of seven notes, you want to have a different one on each letter if at all possible.

The reason is standard notation - if every pitch has a different letter, you can make music a lot easier to read. In standard notation, a note remains the same throughout a measure unless it's altered by an accidental. Once it's been altered, it STAYS altered, unless it's changed by another accidental.

So let's say we're in the key of G. That makes the b6 Eb, and the bb7 Fb (because F is sharp in the key of G, F natural would be b7, and F flat would be bb7).

If you wrote it that way, a measure of alternating eighth notes would be written:

Eb-Fb-E-F-E-F-E-F (the last six notes would be played as Eb and Fb, but you wouldn't need additional flats in the notation)

But if you called the bb7 a natural 6th, every note would need an accidental. I'll use 'n' for the natural sign:

Eb-En-Eb-En-Eb-En-Eb-En

Eight accidentals instead of two. Much harder to read.

But like I said, it depends on context. There are plenty of times a note is written as 6, but it's actually bb7. A good example is a fully-diminished 7th chord.

Diminished 7ths are tertian chords - they're made up of all minor thirds; the formula is 1-b3-b5-bb7. With C as the root, that's C-Eb-Gb-Bbb. But you'll rarely see that chord written in standard notation with a double-flat... because C-Eb-Gb-A is easier to read. Anybody reading the notes will get the right sound; anybody who knows the theory will know it's 'really' Bbb.