The minor scale is the 's darker sibling. It follows the same idea, seven notes built from a recipe of whole and half steps, but the steps come in a different order, and that changes the mood completely.
The simplest minor scale starts on A and uses only natural notes, just like C major does from C:
From here on we leave out the octave note at the top of each scale, so you only see the seven different notes. The last step of each recipe still leads back up to the root.
Counting the steps between its notes gives the minor recipe:
Compare it with the major recipe, W W H W W W H. The half steps sit in different places, and that is all it takes to sound darker instead of bright. This scale is also called the natural minor, to tell it apart from the two variations further down.
Look closely: A minor uses exactly the same notes as C major, it just starts on A. We call A minor the relative minor of C major. Every major scale has one: it starts on the major scale's 6th note, three half steps below the .
That is why the circle of fifths shows A minor right next to C major: they share the same .
Now compare C major with C minor. C minor is the parallel minor of C major: both start on C, but C minor lowers three notes, the 3rd, the 6th and the 7th, by a half step. Play them one after the other to hear the change in mood.
Pick a root note to build its natural minor scale with the same recipe:
In the natural minor, the 7th note is a whole step below the root, so the melody doesn't pull back home as strongly as in major, where the 7th sits just a half step below. The harmonic minor fixes that by raising the 7th by a half step: in A harmonic minor, its 7th note is G♯, just a half step below A.
It's called harmonic because it's mostly used for the chords (several notes played together, which make up the harmony) in minor music; you'll see why in the Chords chapter.
Raising the 7th leaves a big gap of a step and a half (W+H) between the 6th and the 7th, F to G♯. That gap gives the harmonic minor its exotic sound, popular in classical music and metal.
To smooth out that gap, the melodic minor raises the 6th as well: A B C D E F♯ G♯. Classical musicians use it going up and play the natural minor coming down; in jazz it's used both ways.