The available mus. material at the time when the modes became accepted was that which may be nowadays conveniently represented by the white keys of the pf. or org., the notes of which constitute (with slight differences of tuning) the scale worked out scientifically in the 4th cent. bc by Pythagoras and the Gr. thinkers of his time. In the 2nd cent. ad the Greeks were using this scale in 7 different ways: Gr. influence was strong in the early Christian Church and changes in the modal system developed among singers as a practical measure. In the 5th cent. 4 modes were adopted (authentic modes) and at the time of Pope Gregory (c. 540–604) 4 more were added (plagal) and later 4 more, making 12. In the Authentic modes, the 5th note (the dominant), was much used as a reciting‐note in plainsong, and the first (the final), as a cadence‐note, to close a passage. The authentic modes may be re‐created by playing on the pf. octave scales of white notes beginning respectively on D, E, F, and G. A melody played in one of the modes and then in another will alter in some of its intervals and hence in its general effect, as opposed to a melody played in our 12 major or minor scales, which are all alike as to intervals.
The plagal modes were merely new forms of the others, being the same 4 taken in a compass lying not between final and final of the corresponding authentic modes but between their dominant and dominant, the final, on which the cadences fell, thus coming in the middle. In order to avoid having the reciting note at the very top or bottom of the series of notes a new one was chosen, lying 3 notes below the original, and this was now regarded as the dominant. The whole series was now as follows (A = authentic and P = plagal):† The dominants of the two modes so marked (one of them authentic and the other plagal) would normally be B, but this being found an unsuitable note C was adopted instead.
(It will be noted that the odd‐numbered modes are the authentic ones and the even‐numbered the plagal.)
Nearly a thousand years after Gregory a Swiss monk, Henry of Glarus, or Henricus Glareanus, brought forth, in a book called Dodecachordon (1547), a theory that there should, historically, be 12 modes instead of 8. He added modes on A and C (none on the unsuitable B), with their plagal forms, so that the table above was complemented as follows:Glareanus gave his 12 modes what he thought to be their orig. Gr. names and these (though incorrect) have become accepted:It should be clearly understood that the difference between the various modes is not one of pitch but of the order in which fall the tones and semitones. Any mode could be taken at another than its original pitch (i.e. transposed), but in that case its intervals remained as before. Thus the whole series could be set out as beginning on C, when the Dorian and Lydian (to take two examples) would appear as follows: