Adam Clarke on Isaiah 6:3

Holy, holy, holy - This hymn performed by the seraphim, divided into two choirs, the one singing responsively to the other, which Gregory Nazian., Carm. 18, very elegantly calls Συμφωνον, αντιφωνον, αγγελων στασιν, is formed upon the practice of alternate singing, which prevailed in the Jewish Church from the time of Moses, whose ode at the Red Sea was thus performed, (see Exo 15:20, Exo 15:21), to that of Ezra, under whom the priests and Levites sung alternately, "O praise Jehovah, for he is gracious;

For his mercy endureth for ever;" Ezr 3:11. See De Sac. Poes. Hebr. Prael. xix., at the beginning.