Interval Inversion & Compound Intervals
Flipping an interval upside down, and naming the wider intervals that stretch beyond a single octave.
Explainer
You already know interval quality from Grade 2 — major, minor, perfect. Inverting an interval means moving the lower note up an octave (or the upper note down one), which flips its number and quality in a predictable way.
The "Rule of Nine": an interval's number and its inversion's number always add up to 9. A 3rd inverts to a 6th (3+6=9); a 2nd inverts to a 7th (2+7=9).
Quality flips too: major inverts to minor (and back); perfect stays perfect; augmented inverts to diminished (and back).
A compound interval is simply any interval wider than an octave — a 9th is an octave plus a 2nd; a 10th is an octave plus a 3rd.
Worked examples
Inverting a 3rd
A 3rd inverts to a 6th — 3 + 6 = 9, the Rule of Nine.
Major inverts to minor
C to E is a major 3rd. Flip it: E to C (an octave up) is a minor 6th.
A compound interval: a 9th
An octave plus a 2nd — wider than an octave, but built the same way.