One neat property of "perfect" intervals is that, when you invert them, they remain perfect! Perfect unison inverts to unison. Perfect octave inverts to an oct

One neat property of "perfect" intervals is that, when you invert them, they remain perfect! Perfect unison inverts to unison. Perfect octave inverts to an octave. Perfect 4th (C to F) inverts to a perfect 5th! And vice-versa. Contrast to a major third, or major second, which, when inverted, becomes a minor 6th (or minor seventh.) That is, the interval quality of perfects remains the same, where as major/minor/augmented/diminished intervals change qualities.