De institutione musica I.24: Quid sit synaphe.
But in tetrachords thus arranged and constituted there is synaphe, which in Latin we can call conjunction, whenever the middle of a single term continues and joins two tetrachords, as in this tetrachord:
Here, then, is one tetrachord: hypate, parhypate, lichanos, hypate meson; another is hypate meson, parhypate meson, lichanos meson, mese. In both tetrachords, then, the hypate meson is counted, and it is the highest of the upper tetrachord, but the lowest of the later one; and this conjunction is one and the same string, so that the hypate meson, joining two tetrachords, joined the tetrachords hypaton and meson in the description above. Synaphe, then, which is called conjunction, is the middle voice of two tetrachords, the highest of the upper, but the lowest of the later.