Before about 1600, the range of notes used in most European music was more limited than it is today. Nowadays we divide the octave into twelve semitones. If you look on a keyboard (or xylophone etc.) you notice that these are represented by seven white keys and five black keys. The black keys were necessary to develop the full range of major and minor music which came into being after 1600.
The earlier you go before 1600, the less music used “black notes” and concentrated almost entirely on “white notes”. In medieval theory there were eight so-called modes, or “white-note scales” if you prefer. The modes are characterized by (a) having a specific final note (finalis ) ; (b) using all the “white-notes” extending over a range of one octave upwards from the finalis. This range (or ambitus) might also be extended downwards by one note below the finalis ; The four most important modes were:
1. The Dorian Mode: which finishes on the note D:
The FINALIS is the first given note
The OPTIONAL lower note if given neck-down after the rests
The range of the MODE proper is the final 8 notes
3. The Phrygian Mode : which finishes on the note E
5. The Lydian Mode : which finishes on the note F
7. The Mixolydian Mode: which finishes on the note G
Each of these modes has a so-called “plagal” form in which the finalis remains the same, but the range of notes or ambitus is set a fourth (four notes) lower than in the authentic or original form of the mode. To show the relationship of the “plagal” form to the authentic form of the mode the prefix hypo- is used. Compare carefully the modes shown below with those shown above. Notice, too the way the modes are numbered [1,2], [3,4], 5/6], [7/8] relating respectively to the two forms each of Dorian, Phrygian, Lydian and Mixolydian modes.
2. The Hypo-dorian Mode: which finishes on the note D
The FINALIS is the first given note
The OPTIONAL lower note if given neck-down after the rests
The range of the PLAGAL MODE proper is the final 8 notes
4. The Hypo-phrygian Mode: which finishes on the note E
6. The Hypo-lydian Mode: which finishes on the note F
8. The Hypo-mixolydian Mode: which finishes on the note G
