So what exactly are perfect intervals? What makes them so special to be called “perfect”? An interval is an interval, right?
Before we get into the deep logistics of perfect intervals, let's go through a quick review of what an interval in music is first.
Fast Interval Review
In music theory, an interval is described as the distance between two notes. Basically this is describing in greater detail how close or far apart two notes are from each other.
To define an interval, they are called unison, 2nd, 3rd, 4th, 5th, 6th, 7th or 8th (octave).
What is a Perfect Interval?
A perfect interval identifies the distance between the first note of a major scale and the unison, 4th, 5th or octave. Only those intervals can be given the extra attached name as “perfect”.
PU/PP/P1 = Perfect Unison/Perfect Prime
P4 = Perfect Fourth
P5 = Perfect Fifth
P8 = Perfect Octave
In the example listed above, all of the perfect intervals are found within the C Major scale.
Why is it Called Perfect?
That is a very good question that involves more historical analysis into the times of antiquity in order to really answer it properly. From my understanding, the term “perfect” originated due to the musical overtone series.
Apparently perfect intervals ring in a way that other intervals do not. These sound qualities were first discovered and praised in the East.
A guy named Pythagoras was the first person from the West to explore this interesting observation. He is responsible for incorporating it into our current musical language and practice.
The label of “perfect” in addition to a number describes the interval's quality. These intervals are called perfect because the ratios of their frequencies are simple whole numbers.
Another Tool to Use
If one of the interval notes does not include the first note of a scale, it can be a little trickier to find out what the quality of the interval is. Just because the first note of any scale is not included does not mean it could not still be a perfect interval.
The way to find out is to count the number of half steps between the notes.
P1 = 0 half steps
P4 = 5 half steps
P5 = 7 half steps
P8 = 12 half steps
Start with the lower note and count in half steps moving up until you reach the last note. This should help you decide if the interval truly is “perfect”.
Here's some examples of perfect intervals that do not include the first note of the C Major Scale as the bottom note even though they are all still found with the same scale:
A perfect unison is very easy to find because both notes are exactly the same. Likewise, a perfect octave is also simple to detect. Both notes are the same except one of the notes are an octave (8 notes) higher or lower than the other note.
1) Perfect intervals include adding a note above the first note of a major scale that represents the distance of a unison (prime), 4th, 5th or 8th (octave) interval.
2) A perfect interval does not have to include the first note of the major scale. As long as both notes are found in the same major scale, they are considered diatonic intervals. Using half steps as your guide will really help you determine what quality of intervals they are.
3) They are labeled as “perfect” because the sound quality is much different from any other intervals.
That is really all you need to know about perfect intervals! The more you understand the language of music, the better you get at reading and playing!