The following summary may serve as an introduction to the modes, the basis of all organ music through to the seventeenth century. The voice-ranges of the twelve modes are given, the white note being the key-note. All are in their untransposed state with the exception of modes II and XI, which have been transposed respectively up a fourth and down a fifth, reflecting common usage. All pairs of modes function regularly:in the odd-numbered modes Tenor (and Soprano) use the authentic scale, Alto and Bass the plagal scale. In the even-numbered modes the roles are reversed except in the case of mode IV,which is often almost indistinguishable from mode III, having sometimes a smaller range, sometimes a larger one, especially at the upper end of the Soprano. In all cases it is quite normal to extend the voice-range occasionally by one or possibly two notes. For further reading see Meier 1992 and the present writer’s introduction to his edition of Giovanni de Macque’s Ricercari sui Dodici Toni.Unless otherwise stated, the descriptions represent a précis of the widely-known account of the modes given by Gioseffo Zarlino in his Le istitutioni harmoniche of 1558.