The Ison (The Holding Note)

One or more singers hold a single note while the melody moves above it. The ison is not a harmony and not a bass line: it is the tonic of the mode made audible, so that every interval the melody sings is heard against the ground it belongs to. It moves only when the mode moves, and a good isokrates changes it late and quietly. Theologically it is the closest thing the psaltic art has to an argument about itself, since the many voices sing one melody over one unchanging note, and the Church has never wanted a texture in which the voices go their own ways.