In kindergarten and early primary school, children are taught scales as having steps and half-steps, or tones and semitones (T and S). This naming conveys that between one scale note and the next is always “one” something (tone or semitone, step or half step), indicating that these notes are consecutive in the scale despite their different size.
This is great for very young minds but doesn’t reflect how intervals are measured and described in general use.
The smallest unit of musical pitch (in Western culture) is 1 semitone. The semitone is a basic unit, like a millimetre. (Smaller units exist but they measure expression such as vibrato or micro-tuning rather than musical notes). Larger intervals are measured in semitones, not tones and semitones. We say an octave is 12 semitones, not 6 tones or 5 tones and 2 semitones.
(Personally I find that calling 2 semitones a tone is confusing, given that a “tone” is also the American name for a note as well as a word for timbre.)
I never refer to “tones” as an interval size in any of my writing. For scales, I write 2 or 1 (semitones) instead of T or S.
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