Definition:Musical Interval/Historical Note

Historical Note on Musical Interval
The Pythagoreans noticed that aesthetically pleasing musical intervals were obtained by sounding notes from strings whose lengths were in ratios of small natural numbers: $1 : 2$, $2 : 3$, $3 : 4$ and so on.

This may have been one of the inspirations behind their mystical appreciation of the numerical nature of the Universe.