This is the section that had always confused me early on in trying to learn theory. After being instructed that you can’t alter a fourth or fifth interval, we are now learning a prefect interval can be raised or lowered. Meaning, if you raise a perfect interval a half step it becomes augmented or if you lower the perfect interval a half step it becomes diminished. Examples are as shown:
In the key of C, the fourth is the F note. Raising the F a half step to F# results in a augmented 4th.
In the key of G, the fifth is the D note. Lowering the D a half step to D flat results in a diminished 5th.
Most of the time you may not run into using either of the two intervals but it is good tools to learn and keep in your toolbox of theory.
Next lesson we will begin to wrap up the section on interval and move onto scales. Good luck with your playing!