What Is An Interval in Music?
An interval is the distance between two notes, it is labelled by a number and a quality. Intervals can be harmonic and melodic. A harmonic interval is when two notes are played together whereas the notes of a melodic interval are played successively.

What is Interval Quality?
There are five qualities of intervals namely :
- Perfect (P).
- Minor (m or b).
- Major (M or sometimes represented by a triangle ∆).
- Diminished (dim. or bb).
- Augmented (aug. or #).
What is Interval Number?
An interval number represent the position of a note from the root/tonic (R/1) or any other note. These numbers are : 1 (unisson), 2 (second), 3 (third), 4, (fourth), 5 (fifth), 6 (sixth), 7 (seventh), 8 (octave).
Exemple : E is the third of C (the third note after C), G is the fifth, etc. This intervals are called "simple intervals".
It is possible to count past 8, this is what we call compound intervals, they span more than one octave :
- 9 (ninth) actually second + octave.
- 10 (tenth) third + octave.
- 11 (eleventh) fourth + octave.
- 12 (twelfth) fifth + octave.
- 13 (thirteenth) six + octave.
Exemple, A is the thirteenth of C.