Tone

The Tone as an interval measure was already known in Ancient Greece. Aristoxenus (fl. 335 BC) defined the tone as the difference between the just fifth (3/2) and the just fourth (4/3). From this base size, he derived the size of other intervals as multiples or fractions of the tone, so for instance the just fourth was 2½ tones in size.

From a technical perspective, the tone as an interval with frequency ratio 9/8 and a size of ca. 204 cents is exactly the same as the major diatonic second.

see also The measurement of Aristoxenus's Divisions of the Tetrachord