The_Complete_Idiot%27s_Guide_To_Music_Theory

22
Part 1: Tones
Note
An augmented fourth and
a diminished fifth are
enharmonically the same
note.
Here are the three perfect intervals, with C as the root.
Three perfect intervals, starting on C.
Augmented and Diminished Intervals
Okay, now you know that perfect intervals can’t be major or minor. That doesn’t
mean that they can’t be altered, however. You can raise and lower fourths and
fifths—however, the result is not called major or minor. When you raise a perfect
interval a half step, it’s called an augmented interval. When you lower a perfect
interval a half step, it’s called a diminished interval. So don’t call the new
intervals major or minor—call them augmented or diminished.
For example, if you use C as the root, F is a perfect fourth away from the root.
If you sharpen the F, the resulting note (F-sharp) is an augmented fourth above
the root.
Along the same lines, G is a perfect fifth above C. When you flatten the G, the
resulting note (G-flat) is a diminished fifth above the root.
Here are the key augmented and diminished intervals, with C as the root.
Augmented and diminished intervals, starting on C.
Now, just to confuse things, other types of intervals can also be called diminished
and augmented—and these intervals have nothing to do with the perfect
intervals.
To start, you can also create a diminished interval by lowering a minor interval
by another half step. For example, F to D-flat is a minor sixth; if you flatten the
D-flat (yes, there’s such a thing as a double flat), the resulting interval is called a
diminished sixth.
You can also create an augmented interval by raising a major interval by another
half step. For example, F to A is a major third; if you sharpen the A (to Asharp),
the resulting interval is an augmented third.
Fortunately, you don’t have to deal with either type of diminished or augmented
interval that often. But you still need to know what they are, just in case!