[map]

The first six tones appear with their respective syllables initiating the first six phrases.

The 7th tone does not appear as a phrase-initiating syllable. Sancte Ioannes does not even touch tone 7.

The headline is artificially stretching it.

Take it from Guido: hexachords rule.

[Svartalfiar]

The first six tones appear with their respective syllables initiating the first six phrases.

The 7th tone does not appear as a phrase-initiating syllable. Sancte Ioannes does not even touch tone 7.

Right. The SI was added later because the system uses 7 notes but the hymn the sounds were taken from only had 6 of the lines. So in order to finish the system, they had to add a note-word to complete the set and get to 7.