Basic Music TheoryIntroduction
The Major Scale
Primary intervals (unison, 4th, 5th, octave) are considered perfect when the upper note falls within the major scale of the root (the bottom note). If the upper note is raised a half step, the interval is augmented; if the upper note is lowered a half step, the interval is diminished.
For example, consider the following interval:
We can determine that this is a 5th by counting the note names, or lines and spaces, between the two notes. In order to determine the quality of this interval, we need to look at the major scale of the root, which in this case is E. The key of E has 4 sharps: F#, C#, G# and D#.
Here we can see that the top note of this interval, which is B, falls within the E Major scale. Therefore, this is a perfect 5th.
If the top note were lowered a half step, then it would be a diminished 5th.
The following illustration shows an augmented 4th, with F as the root. Tthe 4th note in the F Major scale is B-flat, so the interval from F to B-flat is a perfect 4th. The interval shown below is F to B-natural, making this interval an augmented 4th.
There is a special name for an augmented 4th / diminished 5th (these are enharmonically equivalent): this is called a tritone.
As you can see, the top notes of these intervals, F# and Gb, are the same pitch.
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