In music theory, the term interval describes the distance between notes. We measure this distance in half steps or semitones. Interval names consist of a quality followed by a ordinal number (i.e. Minor 2nd, Major 3rd, Perfect 5th). The way in which intervals are named can be confusing at first so let’s focus on one thing at a time.
The ordinal numbers come from the letter names used in SPN. An interval of a 2nd always consists of two adjacent letters in our musical alphabet (C-D, A-B, and G-A) while a 5th is comprised of notes that are 5 letters apart (C-G, F-C, and D-A).
There are 5 types of interval qualities (major, minor, augmented, diminished, and perfect). The major scale serves as the template for naming these interval qualities
We can understand the other qualities based on Major and Perfect intervals:
Ok, now that all of that’s out of the way, let’s look at Happy Birthday again.
Here’s a sequential list of all of the intervals in Happy Birthday:
If we condense this list to eliminate repeating intervals we get the following:
Out of 24 total intervals here’s the statistical breakdown:
It’s worth noting that vocal melodies, that is melodies meant to be sung, typically contain intervals smaller than a 7th and stay inside the range of an octave. This is especially true of songs meant to be sung by amateurs.