Ionian
Church mode on C, identical with major.
What it means
The Ionian mode runs on the natural notes from C to C and thus corresponds exactly to the major scale: semitone steps between the third and fourth and between the seventh and eighth degrees. It is the only one of the modes that has of itself a leading tone below the finalis.
Musical use
Remarkable is that Ionian did not occur in the original order of the eight church tones. Only in the sixteenth century was it taken into the system together with the Aeolian mode — at a time when practice had long proceeded according to them. Theory here followed the music, not the other way round.
From Ionian and Aeolian, major and minor developed in the seventeenth century as the dominant modes. The reason for this establishment lies in the leading tone: it produces an unambiguous gradient to the finalis and thereby makes the functional cadence possible. Modes without a leading tone had first to obtain it through musica ficta; Ionian had it from the start.
In the description of modern modal music — in jazz, in folk, in popular music — the name is still used when the modes are enumerated as a system. There Ionian designates simply the major scale in modal view.
Ionian is to be distinguished from Lydian, which differs only in the raised fourth degree, and from Mixolydian with its low seventh degree.
Glarean described Ionian in 1547 in the Dodecachordon as the eleventh and twelfth modes, together with its plagal form Hypoionian. Theoretically it was a latecomer, in practice it had long been widespread in secular music. From it the major scale emerged. In modal jazz theory, Ionian designates the scale over the tonic chord with a major seventh.
Performance practice
If you meet the name in scale tables, read it as major — in substance there is no difference. In working with early music it is worth looking at whether a piece is already conceived as Ionian or still modal; the cadence formation usually gives it away. And practise modes by comparison, playing them one after another on the same root; in this way the differences stand out.