5 Oct 23:44
A third + an octave is called a tenth.
it’s not ten semitones, it’s an interval according to the definition of “octave”.
Ten semitones is a “minor seventh”, one tone smaller than a perfect octave.
A minor 10th is 15 semitones, a major 10th is 16.
“why do we consider “major 3rd + an octave” the same thing as a “major 3rd”?”
It’s called octave equivalance. In a certain type of analysis both are given the same number.
If your lowest note = 0, any kind of major 3rd, in other words a major 10th or …
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