The major scale is not very difficult to understand. The pattern is, whole step, whole step, half step, whole step, whole step, whole step, and half step. See Pic 1 for the key of C.
If I follow that same pattern starting on a G, the root note of the key of G, it creates an F#, which is why the key of G Major has the F# that you see in Pic 2. Pretty straight ahead, right?
So far, so good. Every major however has a relative minor, meaning a minor scale that shares its key signature. The starting note is the sixth tone of the major scale so for the key with no sharps or flats, an A. This gives me a flatted 3rd (and flatted 6th and flatted 7th), making it sound like a minor scale. Still no biggie, right? See Pic 3.
Ah, but here is the rub: that is only one of three kinds of minor scales that share the major scale’s key signature, the natural minor. (It is also known as the Aeolian mode.)
It evolved over time. The natural minor came first, and when you listen to a Gregorian chant, that is what you usually are hearing. That was fine for simple melodies but when composers like Bach wanted something more harmonically sophisticated, and with the strong feeling of finality a major I, IV, V cadence gave them, it was not cutting it. So they then introduced the harmonic minor scale, which raised the 7th tone a half step, as you see in Pic 4.
This is very common in Eastern European music. Think any song from “Fiddler On The Roof.” Also The Beatles “Girl”. The line “all about the girl who came to stay.”
Great, so then there were two, but now a third? Yep.
Introducing (tada!) the melodic minor. For more complex melodies later composers were not happy with either of those and even worse, they liked it one way ascending and another descending. This gives us the flatted 3rd only on the way up, while on the way down, it is the same as the natural minor.
So in Pic 5, you see an ascending melodic minor scale, while to see the descending melodic minor, just look back at the natural minor scale.
Some famous songs that feature the melodic minor would be The Beatles, again, with “Yesterday”. The line “all my troubles seemed so far away.”
Have fun when you listen to songs by trying to identify these three kinds of minor scales!