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

- C-C = Perfect Unison
- 0 Semitones

- C-D = Major 2nd
- 2 Semitones

- C-E = Major 3rd
- 4 Semitones

- C-F = Perfect 4th
- 5 Semitones

- C-G = Perfect 5th
- 7 Semitones

- C-A = Major 6th
- 9 Semitones

- C-B = Major 7th
- 11 Semitones

- C-C = Perfect 8th (Octave)
- 12 Semitones

We can understand the other qualities based on Major and Perfect intervals:

- Major – 1 semitone = Minor
- Major – 2 semitones = Diminished
- Major + 1 semitone = Augmented
- Perfect – 1 semitone = Diminished
- Perfect + 1 semitone = Augmented

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*:

- G4-G4 = Perfect Unison
- G4-A4 = Major 2nd Ascending
- A4-G4 = Major 2nd Descending
- G4-C5 = Perfect 4th Ascending
- C5-B4 = Minor 2nd Descending
- B4-G4 = Major 3rd Descending
- G4-G4 = Perfect Unison
- G4-A4 = Major 2nd Ascending
- A4-G4 = Major 2nd Descending
- G4-D5 = Perfect 5th Ascending
- D5-C5 = Major 2nd Descending
- C5-G4 = Perfect 4th Descending
- G4-G4 = Perfect Unison
- G4-G5 = Perfect Octave Ascending
- G5-E5 = Minor 3rd Descending
- E5-C5 = Major 3rd Descending
- C5-C5 = Perfect Unison
- C5-B4 = Minor 2nd Descending
- B4-F5 = Diminished 5th Ascending
- F5-F5 = Perfect Unison
- F5-E5 = Minor 2nd Descending
- E5-C5 = Major 3rd Descending
- C5-D5 = Major 2nd Ascending
- D5-C5 = Major 2nd Descending

If we condense this list to eliminate repeating intervals we get the following:

- 5 Perfect Unisons
- 3 Minor 2nds
- 7 Major 2nds
- 1 Minor 3rd
- 3 Major 3rds
- 2 Perfect 4ths
- 1 Diminished 5th
- 1 Perfect 5th
- 1 Perfect Octave

Out of 24 total intervals here’s the statistical breakdown:

- 21% Perfect Unisons
- 12.5% are Minor 2nds
- 29% are Major 2nds
- 4% are Minor 3rds
- 12.5% are Major 3rds
- 8% are Perfect 4ths
- 4% Diminished 5ths
- 4% Perfect 5ths
- 4% Perfect Octave

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.