Perfect Intervals: Unison, 4th, 5th & Octave
Four intervals get a special name — "perfect" — because their frequency ratios are the simplest in all of music, and they've sounded stable for over two thousand years.
The simplest ratios in music
Every pitch is a frequency, and every interval is a ratio between two frequencies. The perfect intervals have the simplest ratios possible: the unison is 1:1 (identical), the perfect 4th is 4:3, the perfect 5th is 3:2, and the octave is 2:1. Simple whole-number ratios like these produce sound waves that line up often and predictably — which is exactly what our ears hear as stability and consonance.
A history of stability
Ancient Greek theorists already recognized these ratios as special, and centuries later medieval European harmony (organum) was built almost entirely from unisons, 4ths, 5ths, and octaves — 3rds and 6ths were considered too unstable to use as points of arrival until much later in music history. That's the legacy behind the name "perfect": not "flawless," but harmonically fixed and unshakeable.
Common questions
Why are they called "perfect" intervals?
Because their frequency ratios — 1:1, 4:3, 3:2, and 2:1 — are the simplest possible, which theorists considered the purest, most stable relationships between two pitches.
What are the four perfect intervals?
The perfect unison, perfect 4th, perfect 5th, and perfect octave — the only interval numbers that ever use the word "perfect."
Why were perfect intervals prized historically?
Their simple ratios made them the most consonant intervals available, so early harmony leaned almost entirely on them before 3rds and 6ths became acceptable.
Are perfect intervals always consonant?
The 5th and octave are almost universally consonant. The 4th is consonant melodically but is treated as a mild dissonance in some strict classical part-writing contexts.
Hear stability for yourself
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