The distance between two notes is classed as an interval. An interval played as one note after the other is called a melodic interval. All melodies are just a series of intervals ascending and descending.
If two notes are played together, it is called a harmonic interval. Chords are built from intervals played at the same time and this is what creates harmony.
When notes are used from a specific key, the intervals created are referred to as diatonic.
In a major key, the diatonic intervals are created from playing the tonic or 1st note of the major scale and following it with another note from the scale.
The diatonic intervals using notes from a major scale are:
1st note – 1st note
1st note – 2nd note
1st note – 3rd note
1st note – 4th note
1st note – 5th note
1st note – 6th note
1st note – 7th note
1st note – 8th note
An interval is determined by its number position and its quality.
2nds, 3rds, 6ths & 7ths are Major.
Unisons, 4ths, 5ths & Octaves are Perfect.
See my video “Intervals Part 1” for an explanation on how to apply this.