Ionian.

The name assigned by Glarean in the Dodecachordon (1547) to the authentic mode on C, which uses the diatonic octave species c'–c'' divided at g' and consisting of a fourth species of 5th (tone–tone–semitone–tone) plus a third species of 4th (tone–tone–semitone), thus c'–d'–e'–f'–g' + g'–a'–b'–c''. With this octave species identical with the C major scale, the Ionian mode, together with its plagal counterpart, the Hypoionian, is essentially the same as the major mode of tonal music (see Tonality), in which the dominant lies a 5th above the tonic, or principal scale degree.

In the Middle Ages liturgical songs with a final written at c' were regarded as mode 6 transposed up a 5th. There were in principle no medieval modal forms with finals on C, but certain late-composed chants in mode 5 (e.g. the Marian antiphon Alma redemptoris mater) were often written a 4th lower in the Guidonian diatonic system, making them into authentic pieces on C. It is said that the C mode was called tonus (or modus) lascivus, the ‘frolicsome’ or ‘wanton’ mode, in the Middle Ages and considered apt only for secular music. However, there seems to be no direct evidence for this expression in theoretical sources. It appears to originate with Glarean's own reference to the ‘lascivam petulantiam’ (‘frivolous wantonness’) that the Ionian mode possessed according to the ancients, and with his use of the terms lasciva and lascivia to designate the ‘wanton’ use of B in chants of modes 5 and 6 (F modes) and 1 and 2 (D modes): this had the effect, in terms of his putatively reconstructed 12-mode system, of converting them from Lydian–Hypolydian and Dorian–Hypodorian to Ionian–Hypoionian and Aeolian–Hypoaeolian transposed, respectively.

Glarean also classified polyphonic music according to 12 rather than eight modes, and regarded pieces composed round a tonal centre of F and set in cantus mollis (i.e. with a one-flat signature) as embodying the transposition of the Ionian or Hypoionian mode; most 16th-century musicians, however, seemed to consider them as embodying modes 5 and 6 of the traditional set of eight, which from the very beginnings of medieval modal theory had required the prevalence of b over b for their fourth degree above the final, f; thus b corresponds to f' in the Ionian mode (see Lydian and Hypolydian).

HAROLD S. POWERS/R