MODAL TONALITY Tonality is the element which groups a succession of musical sounds intelligibly round some centre. With the development of polyphony, tonality becomes as important as the concord-discord system itself ; and, indeed, that system could not have existed without tonal guidance at every point. Discord is transition; concord is finality. The task of tonality is to organize various degrees of finality among concords. The first decision made by pure polyphony (but revoked in a later age) was that the minor triad, though it might be a concord, could never be final. A bare sth or even a bare octave would be more acceptable, as being a potential major triad. The final chord, whether complete or not, requires to be approached by chords in a well-defined relation to it. Two types of full close, or cadence, thus came into existence —the authentic, in which the final chord is preceded by a major chord whose bass is a 4th below or a sth above the final bass— and the plagal, in which the penultimate chord is based a 4th above or a 5th below the final, and is major or minor according to the mode : The modes were named after those of ancient Greece, wrongly identified in particular; and theory clung to terms derived from non-harmonic notions long after the practice of composers had become inveterately harmonic. An aesthetically-correct account of Palestrina's tonality is much more easily achieved by a descrip tion in terms of Beethoven's key-system than by any attempt to refer it to the orthodon modal theory.
According to the finally-prevalent statement of that theory there were ideally 14 modes, two based on each degree of the diatonic scale. Practically the modes based on B were impossible, as the diatonic 5th from B is imperfect. The numbers of these imaginary modes, XI. and XII., were piously retained for them, together with the name of Locrian. The "authentic" modes ran from the "final" or fundamental note to its octave. Each authentic mode was allied to a "plagal" mode, having the same final, but lying a fourth lower. This is an important distinction in purely melodic music and can be clearly recognized in folk-songs. Thus, The Bluebells of Scotland is authentic, while Auld Lang Syne is, except for an isolated top note, typically plagal. In polyphonic music the difference between an authentic mode and its plagal companion is a vague matter settled by the position of the tenor voice.
The word "modulation" was used in the theory of modal music to denote the formation of full closes on other notes than the final. The i6th century composer developed a perfect sense of key around his cadences, and he knew very well what he was doing when he avoided stimulating that sense elsewhere. He selected his subordinate cadences on no more cogent principle than the avoidance of monotony. He was like a painter whose draughts manship is faultless in faces and figures, but who sees no objection to implying a different horizon for each detail in his picture. And harmony has no such relation to external nature as can justify critics in calling modal tonality archaic. Palestrina's tonality is one of the most mature and subtle things in music, and later developments cannot lessen its truth to the nature of art.
Here are the 12 modes which theoretically underlie the tonality of the i6th century. Every composition was written in one of these modes, and its incidental modulations were not regarded as visits to another mode, though that is aesthetically what they really were. The diagram gives the name of the authentic position above each scale, and the plagal name and position below. The white note is the final. The imaginary Locrian modes (with B as final) are omitted.
In practice these modes are not always easy to ascertain. The B natural in Lydian tonality is so difficult to handle that the great masters almost always flattened it permanently and put the flat as a key-signature, thus producing an Ionian mode transposed, or plain modern F major. (All modes could be thus written a 4th higher; and apart from this, the actual pitch of performance was determined by convenience and was bound by no fixed standard.) The Phrygian mode cannot form an authentic cadence ; and its plagal cadence (shown in Ex. lob) sounds to our ears like a half close on the dominant of A minor. This is quite final enough for modal harmony; but a very slight impulse may make Palestrina reverse the cadence and so end with a chord of A. This does not make the mode Aeolian, and, though the Aeolian mode looks as if it was the origin of our minor scale, true Aeolian polyphony is of all harmonic styles the most remote from modern music. The Dorian and Phrygian modes are much nearer to our notions of a well-grounded minor key. The Ionian mode is identical with our major key; and Mixolydian tonality is like a major key with either an excessive emphasis on the subdominant or a top-heavy and finally prevalent dominant.
Extraneous sharps constantly come into modal music through the necessity of providing major penultimate chords in authentic cadences, as well as final major 3rds for minor modes. Flats were no less often necessary to correct the tritone 4th between F and B (hence the shape of the flat, and Morley's naming of it as the B clef) . The rules governing these accidentals were so well known that singers resented the providing of the signs where the need of such musica ficta was self-evident. On the other hand, many of the most mystical harmonies, such as the opening of Pales trina's Stabat Mater, were the gifts of creative imagination equally remote from modal theory and modern tonality. Brahms under stood modal harmony much better than the critics who blame him for violating the modes of folk-songs by not setting them in a kind of musical Wardour street. If you want to set old tunes without using leading-notes and changes of key you must not harmonize them at all.