The “Rule of 9” Trick to Learn Intervals

Intervals are an easy concept to understand. But if you don't understand them, it can be very frustrating as an aspiring piano player. You may be aware that intervals are simply the distances between notes. Many beginning piano players aren't aware that you can have ascending or descending intervals.

But because intervals are asymmetrical, they change as you invert them. In other words, any given note that's a particular interval above another note will not be same interval below it.

For example, if we have C with G above it, this is known as the interval of a 5th.But if you have the same C but have the G below it, it's no longer a 5th. It's an interval of a 4th. It's the same note, it's just inverted which changes the interval.

Just to be clear, I'm strictly talking about intervals here and not keys of music. C and G is found in multiple keys, so it doesn't matter what key any two notes may be in. The intervals are the same and that's all I'm talking about in this article.

For example,

If I have a C Major 7 chord, C to G is a perfect 5th.

If I have an A flat Major 7 chord which also contains C and G, those two notes remain a perfect 5th even though their number sequence differs from the C Major 7 chord.

So what is the rule of 9 trick and how is it helpful?

The definition of the rule of 9 trick is simply this: When any interval is inverted, the sum of the ascending and descending interval adds up to 9. This is especially helpful to understand and remember intervals to take your musical skill to a higher level of playing.

Getting back to our earlier example of 4ths and 5ths inverted, they equal 9 when added together.If you have a 2nd interval up and invert it, it becomes a 7th interval down and vice versa.The sum total of both 7+2=9.

If you have a 3rd interval up and invert it, it becomes a 6th interval down and vice versa.The sum total of both 3+6=9.

To determine an inverted interval, simply ask yourself: What number added to the interval you're starting with equals 9? The answer will be the interval above or below it respectively.

Interval Qualities

If you're familiar enough with intervals, you're aware of interval qualities. This is essentially the type of intervals that any given interval may have (Major, minor, perfect etc).

There are 5 types of interval qualities

Perfect
Diminished
Augmented
Major
minor

Just as intervals change as you invert them, so do their qualities and it's equally as simple to determine the interval quality.

If the interval is Perfect, it remains perfect when inverted.
If the interval is Major, it becomes minor when inverted.
If the interval is minor, it becomes Major when inverted.

Now here are the outlying inverted interval qualities. These affect augmented and diminished intervals.

If an interval is an Augmented 5th, it becomes a Major 3rd when inverted.
If an interval is a diminished 5th, it can be referred to as a diminished 5th or an augmented 4th when inverted.
If an interval is an Augmented 4th, it becomes a diminished 5th when inverted.