Modal Harmony, Part I

The kind of harmony taught in most college and university music courses deals with what is known as the "common practice" era, i.e. the kinds of things the majority of composers did during the period approximately 1650-1850.
After 1850 composers started to experiment more with harmony, but prior to 1650 the major/minor system was not in use and in its place a system of modes was employed to construct diatonic melodies.
Today an understanding of modal harmony provides the organ scholar with many advantages, not just in the analysis of early music but also because we find the modal harmonic system as used during the Renaissance still employed today in composition and improvisation.
This modal system derives from ecclesiastical chant and traces its roots through the following developments:
In the late 4th century Ambrose, bishop of Milan, defined 4 modes beginning on D and moving upward to G to which he gave numbers; the nature of each one of these Ambrosian modes was defined by the position of the semitones (half steps) in the scale:
1st tone ... d e f g a b c d
2nd tone ... e f g a b c d e
3rd tone ... f g a b c d e f
4th tone ... g a b c d e f g
This system was modifed under the direction of Pope Gregory (pope in the period 590-604) by increasing the number of modes to 8.
The 4 additional modes were produced by starting each of the Ambrosian modes a 4th lower, so that the keynote or final note appeared in the middle of the scale.
These 8 separated into 2 groups: the 4 ancient Ambrosian modes were called authentic, and the new modes were called plagal:
1st tone ... authentic ... D e f g a b c d
2nd tone ... plagal ... a b c D e f g a
3rd tone ... authentic ... E f g a b c d e
4th tone ... plagal ... b c d E f g a b
5th tone ... authentic ... F g a b c d e f
6th tone ... plagal ... c d e F g a b c
7th tone ... authentic ... G a b c d e f g
8th tone ... plagal ... d e f G a b c d
This was the system specified for Gregorian chant.
It used only what we would now think of as the white keys of the piano and consisted of only a single melodic line.
Around 1020 the Benedictine monk Guido of Arezzo (c. 995-1050), who spent his life in Italy studying and writing about all that was known about music up to that time, and while other theorists were still working with the 4-note scale (tetrachord), he gave names to a modal hexachord (6 scale degree) pattern he established, which was another step toward the modern diatonic scale of 7 steps.
Among the many other innovations he introduced to music (including the most formidable achievement of the introduction of a practical system of musical notation), Guido compiled a set of names for a 6 tone (hexachord) scale pattern matched to the 1st syllables of each line in the Hymn "Ut queant laxis" composed by Paul the deacon c. 774 for St. John the Baptist Day:
UT queant laxis
REsonare fibris
MIra gestarum
FAmuli tuorum
SOlve polluti
LAbii reatum
Sancte Johannes.
Thus the names of the first 6 scale degrees became UT, RE, MI, FA, SO, and LA.
In Guido's time this became an extremely practical aid to singers; by associating the names of the notes of the scale with these syllables, singers would have a clear idea of how each note of the scale was supposed to sound in relation to all the other notes.
Once musicians began to work with a diatonic scale of 7 notes, SI as the name for the 7th scale degree was not agreed upon until the late 1600's (the "S" was taken from the S of Sancte and the "I" from the old J, written like an I, from Johannes); in some Western countries this SI has become the syllable TI.
The use of the more sonorous syllable DO was substituted in place of UT around 1673 by Giovanni Maria Bonocini, but this practice, to this day, has not been universally adopted.
This system of solmization is in widespread use today.
As polyphony developed through the Middle Ages the tritone between B and F became a problem (because of its imperfect 5th); this was solved by the introduction of the note Bb, which originally was not thought to be a different note, as we do today, but as a modification of the original note B.
As time passed additional modifications were allowed, although the strict church system still allowed only Bb.
The system we use today is based on the compositional practice of the Renaissance period.
In the 16th century Glareanus assigned Greek names to each of the existing modes which were not connected to any system actually in use in ancient Greece.
The distinction between plagal and authentic has for the most part been abandoned in favored of the Renaissance system of 7 modes (photo):
IONIAN ... c d e f g a b c ... scale pattern 1 2 3 4 5 6 7
DORIAN ... d e f g a b c d ... scale pattern 1 2 -3 4 5 6 -7
PHRYGIAN ... e f g a b c d e ... scale pattern 1 -2 -3 4 5 -6 -7
LYDIAN ... f g a b c d e f ... scale pattern 1 2 3 +4 5 6 7
MIXOLYDIAN ... g a b c d e f g ... scale pattern 1 2 3 4 5 6 -7
AEOLIAN ... a b c d e f g a ... scale pattern 1 2 -3 4 5 -6 -7
LOCRIAN ... b c d e f g a b ... 1 -2 -3 4 -5 -6 -7
Thus, with a key signature of no sharps or flats, 7 possible modes, or scale patterns, can be represented (the Locrian mode, because the 5th degree of its scale is not perfect, is of theoretical interest only).
Any starting note can be chosen -- every key signature can generate 7 possible modes; in the Renaissance however, not all notes would have been allowed as starting notes.
When these 7 Renaissance modes are tabulated using C as the starting note, each mode starting on C will have a different key signature:
C Ionian ... C D E F G A B ... no flats or sharps
C Dorian ... C D Eb F G A Bb ... 2 flats
C Phrygian ... C Db Eb F G Ab Bb ... 4 flats
C Lydian ... C D E F# G A B ... 1 sharp
C Mixolydian ... C D E F G A Bb ... 1 flat
C Aeolian ... C D Eb F G Ab Bb ... 3 flats
C Locrian ... C Db Eb F Gb Ab Bb ... 5 flats
In early chant we find the most commonly used modes being the Ionian (same as major scale), Dorian (with minor 3rd and major 6th), and Mixolydian (with minor 7th); the Aeolian is not too uncommon, but the Lydian and Phrygian are extremely rare; the Locrian is never found.
In the Baroque period all modes except the Ionian, Dorian, and Aeolian were pretty much discarded by composers; the Ionian and Aeolian were renamed the major and natural minor scales, respectively, however, in actual use, the Aeolian began to be modified by the use of a raised 7th degree leading tone, creating the so-called harmonic minor scale.
This change derived from the medieval and Renaissance use of "musica ficta" -- adjustments made to certain notes (what we now know as accidentals) -- to avoid forbidden harmonic relations such as the tritone (augmented 4th) or for other reasons dealing with vertical harmonic relationships.
Since traditional chant consisted of a single melodic line only, without harmony, musica ficta was never used, hence raised leading tones in the Dorian and Aeolian modes were never encountered in chant.
Since the augmented 2nd interval resulting from raising the 7th degree of the natural minor scale to create a leading tone was found difficult for singers, Baroque composers also made a further modification to the harmonic minor scale by also raising the 6th scale degree a half step in ascending; this created the so-called melodic minor scale.
This same melodic minor scale, in descending, reverts to the natural minor scale and simply follows the key signature of the passage.
(con't in Part II)