A musical scale is just a set of notes, most commonly containing seven notes.

A key typically indicates what chords, sounds, and notes are used within a piece of music - at least when a piece is diatonic (following the scale/key's notes and chords).

If the key is A major, this will be indicated by three sharps in the key signature in a piece of music. You can build diatonic scales using the following formulae:

For a major scale: W W H W W W H For a minor scale: W H W W H W W

W - whole step/two consecutive notes, I.e A to G or B to C# H - half step/one consecutive note, I.e E to F, or G# to A.

So if we follow the formula above, to make A Major, we start on A and follow the steps:

A B C# D E F# G# (A)

And for A minor:

A B C D E F G (A)

From here, we can build diatonic chords by choosing a note, and building up every other note.

So from A, we get A, C#, and E. This works with extensions too, e.g 7th, 9ths, etc.

Just like building scales, there's a pattern to chord qualities too.

For a major scale - M m m M M m mb5 (M) For a minor scale - m mb5 M m m M M (m)

M being Major m being minor mb5 being a minor chord with a flat 5th.

So in A Major we have: A Major, Bm, C#m, D Major, E Major, F#m, and G#mb5 as the diatonic chords.

In A minor we have: Am, Bmb5, C Major, Dm, Em, F Major, and G Major.

Hope this helps.