In our “Intro to Intervals,” we learned how to recognize major, minor, and perfect intervals—first, recognizing the distance between the two notes on the staff and then counting the precise number of steps between the two pitches. In addition to major, minor, and perfect, there are two other interval qualities that can come into play: augmented and diminished.
Any perfect interval or major interval becomes augmented if it is made one half-step (semi-tone) larger. An augmented interval is written with a capital “A”—for example, an augmented fifth is A5. (You may also see it denoted with a plus-sign before the interval number.)
With the exception of unisons*, any perfect interval or minor interval becomes diminished if it is made one semi-tone (half-step) smaller. A diminished interval is written with a lowercase “d”—for example, a diminished third is d3. (You may see also it denoted with a small circle before the interval number.)
The space between the pitches can be made larger or smaller by either one of the two notes being raised or lowered. However, keep in mind that the notes do not actually change position on the staff; instead, the pitch is raised or lowered using accidentals—flats, sharps, or even double-flats or double-sharps if needed.
As an example, let’s create a diminished fourth up from D. First, find the perfect fourth by counting up four notes, starting on and including D—D, E, F, G.
Now, to make this perfect interval diminished—or smaller—lower the G to Gb. (Another option would be to raise the D to D#.)
In the diminished fourth example above, notice that the distance from D up to Gb is four half-steps. Interestingly, the distance from D up to F# is also four half-steps—and sounds exactly the same—yet that interval is a major third. For that reason, spelling matters!
There are always multiple ways to write, or spell, notes while ending up with the exact same sound. So why do composers and songwriters bother using augmented and diminished intervals—seemingly the more complicated option? In other words, why not simply write a major third rather than a diminished fourth?
Mostly, the reasons for this have to do with established rules for placing notes correctly in the context of the key signature and in the progression of a harmony. Plus, the way that intervals are “spelled” can subtly impact how the music is interpreted by a performer.
*BRAIN TEASER: Why is there no such thing as a diminished unison? Because diminishing an interval, by definition, makes it smaller—but when you lower one note of the unison, the interval has grown larger, not smaller!