Properties of Perfect Intervals

The intervals of a fourth, a fifth, and an octave/unison. These numbers continually show up again and again in tonal music. They do not imply a major or minor quality, but why call them “perfect”?

  • they contain an open or hollow sound
  • unlike non-perfect intervals, perfect intervals can only be augmented or diminished
  • when inverted, a perfect interval remains perfect. All other types are flipped.
  • they appear at the beginning of the harmonic series, therefore are extremely consonant
  • they are nearly identical in the just intonation and 12TET tuning systems
  • the tritone, an extremely dissonant interval of either an augmented fourth or diminished fifth, lies directly between both P1 and P8, and P4 and P5
  • really… can you deny perfection when you see it?
  • they perform the functions of the tonic, dominant, and subdominant scale degrees and are therefore crucial in establishing tonality
  • most of these do not in fact explain what makes them stand out but are rather direct results of them being chosen in the first place

That’s not nearly as much as I thought. Think of anything else?

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