Intervals are sound differences between two notes. We measure how many steps are between two notes, or their distance. Start from a scale's Root note, and any other note is an interval that is steps away.
Starting from C in the C Major scale:
Intervals have different names based on their sound difference. Name them starting from the lower to the higher note. With scales, it's always the 1st note, the root, that's the lower note. Minor scale intervals are explained later.
The Major steps above, simplified:
Now, for the a minor scale (the relative minor - same key signature as C Major):
Notice the difference of interval names, between the Major and minor scales: The 3rd, 6th, and 7th interval names match the type of scale, Major (CDEFGABC) or minor (abcdefga).
|Major||P1 | M2 | M3 | P4 | P5 | M6 | M7 | P8|
|minor||P1 | M2 | m3 | P4 | P5 | m6 | m7 | P8|
However, the 2nd note in the minor scale is a Major interval:
Again, the 2nd interval, whether a Major or minor scale, is a Major 2nd. The half steps remain the same, 2 half steps. But the 3rd, 6th, and 7th intervals, have different names because of different half steps between (Major or minor).
Next, compare the A Major scale to the a minor parallel scale:
|A  B  C#  D  E  F#  G#  A|
|a   b   c   d   e   f   g   a|
Parallel minor scales have the 3rd, 6th, and 7th notes flat from Major scales. So a Major interval becomes a parallel minor when we flat those intervals. Comparing the A Major and minor scale:
...with c,f,g being the minor scale intervals. It's easy now to switch from a Major to a parallel minor key. Just flat the 3, 6, and 7th notes of the Major scale. (To flat a sharp note, lower it a half step by removing the sharp.)
On the Flat side, here's the Bb Major Scale:
Its Parallel minor scale is:
As before, the 3, 6, 7th notes of the parallel minor are flat (a half step below), from the Major scale. In this bb minor scale, there are 5 flat and 2 natural notes. Simply remember to play c and f natural, then play all else flat.
Recommended reading: Relative and Parallel minors
Major or minor intervals may change with each scale step (12345678).
However, for both Major and minor scales, a Perfect interval always remains a P4 or P5 (it's perfect like that), and the first and last notes are always the same (P1 or P8).
We may create more interval types when we further sharp or flat notes in a scale (making the interval notes farther apart, or closer together).
More intervals are Augmented and diminished. To produce these, either sharp a Major interval (Augment it), or flat a minor interval (diminish it).
For both Major and minor scales:
When the interval identification gets confusing, we may listen to the sound of the root compared to the upper note. Intervals are really just half steps, added up. Thinking in terms of steps allows us to make an interval anywhere between two notes. We may flat or sharp a note to change its interval into a Major, minor, Perfect, Augmented, or diminished.
To hear all of the intervals in a scale, play a root note, together with each of its 7 individual scale notes. Then, play the root and next note individually, to hear how far apart in sound they are.
For practice listening to intervals, find a simple song to hear its melody's intervals. For example, the song, "Row Row Row Your Boat" has Perfect unisons (Row, Row, Row), a Major 2nd (Row, Your), and a Major 3rd (Row, Boat). It's also the first three notes of a Major scale (row your boat).
Sharps and flats space out the intervals of notes in a melody. As background chords change, the melody intervals remain the same, but melody notes are sharped or flatted according to the new key of the chord.
Regarding intervals, some musicians focus on certain sound patterns that distinguish their style. This freedom of choice, and the skill to bring it about, is what makes music fun!