Talk:Common tone (scale)

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Recommended correction re: the tritone[edit]

The sentence "However many times an interval class occurs in a diatonic scale is the number of tones common both to the original scale and a scale transposed by that particular interval class" holds for many cases, but not all: the number of common tones between a set and its tritone transposition will be double the number of tritones in the set. For example, the Eb-major scale transposed by a tritone yields the A-major scale. Both of these scales are diatonic, and each contains a single tritone. However, these two scales have two pitches in common: pitches D (pc 2) and G# or Ab (pc 8), assuming enharmonic equivalence. For another example, the octatonic scale contains 4 tritones, and an octatonic scale and its tritone transposition are the same, which means 8 common tones are shared.

I recommend trimming the next sentence to "For example, then, modulation to the dominant (transposition by a perfect fifth) includes six common tones between the keys as there are six perfect fifths in a diatonic scale," and including a caveat about the tritone. One may elect to say a few more things about this: this exception is due to the tritone's higher transpositional symmetry; the number of common tones is doubled in particular because the tritone transpositional symmetry is 2; this can be generalized to other embedded subsets of sizes greater than 2. Scottbmurphy (talk) 16:59, 11 October 2023 (UTC)[reply]

OK, now that I look at it closer, I see that the article explicitly does not assume enharmonic equivalence. But an interval-class vector does, unless stated otherwise, so there's still a departure from conventional theory here. — Preceding unsigned comment added by Scottbmurphy (talk • contribs) 18:52, 11 October 2023 (UTC)[reply]