The objective standard which musicians in earlier times used as their starting point was a series of notes which actually exists in nature. These notes can be observed when something long, thin and stretched - a twig, or a creeper, perhaps - is set in motion by the wind, and the wind varies in strength: you get the same series, relative to the starting note, if you 'blow a raspberry' down a simple tube and increase your lip pressure (like a brass-player does), if you touch a stretched, vibrating string at various points (like a violinist or guitarist does to produce 'harmonics'), and even if you whirl a long flexible tube around your head and vary the speed.

If you write down these notes, and arrange them in order going upwards (they could also go downwards), like steps on a ladder ('scala' in Greek and Latin), starting with a low F, it looks like this:

ex.1: the natural 'harmonic series'

A physicist can demonstrate that what a listener considers to be a single note normally includes all these other notes above it as well: since they are higher than what the listener considers to be 'the note', they are called 'overtones'. If you listen carefully you can sometimes hear the lowest of them - above the bottom note of a resonant guitar, or a fruity low note on a piano, for instance: they vary in strength, the higher they are, the weaker they sound.

It is surely no coincidence that there is a parallel to be seen in the history of harmony: in the Middle Ages the intervals used as the basis of harmony were those between the first four notes of the natural series, in the Renaissance the next two were used, by the middle of the 18th century the seventh of these notes was normal, and since the later 19th century all the others have been used.