Term used in Mediaeval music for those church-modes that lie a quarter below the Authentic modes; A melody is called plagal if the tonic lies a quarter above its lowest note (not counting a leading tone); Metaphorically, the 16-17th century Hümmelchen, the Gajdas of the Balkans and the Bock types of Central Europe, &c. may be called plagal bagpipes; ➺ Authentic.