The Comparative Cycle of Octoechos

In English from here - and here .
Actually, for Legetos, the real base of the pentachord is Di, the upper note in the Ni-Di pentachord (tetraphonia).
Legetos is in fact, Agia, but Agia has a upwards movement towards Di, so you have to go down to go up. So Di-Ga-Vu-Ga-Di.

Above the Di, you can sing anything, either chromatic or diatonic, or enharmonic even, because the border of Echos Tetartos is the Di. The mode's scale isn't on an octave, but only on the pentachord Ni-Di, and from Di upwards, you are no longer in the scale of Echos Tetartos.
That's why some apolytikia have the soft chromatic phthora on Di, and the base on Di, but are still Echos Tetartos, because what truly matters is the scale below the Di.

The most correct isokratima for diatonic Legetos should then be Di, same as in the Chromatic Tetartos.
 
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