The Modes, as we know them today, have developed over centuries from the Greek Scale worked out scientifically, in the 6th Century B.C., by Pythagoras and other Greek mathematicians of his time. By the 2nd Century A.D., the Greeks had developed their scale into seven modes.

In the 4th Century A.D., St. Ambrose, Bishop of Milan, organized the Church music into an orderly system. His system was based on the Greek Scale, but set out in four Modes, beginning respectively on D, E, F, and G (with no sharps or flats). At the end of the 6th Century, Pope Gregory I (the Great: c. 540-604; pope in Rome 590-604) revised the Church Plainsong and added four Plagal Modes to the four Ambrosian Modes, which he called the Authentic Modes.

In Modes, what we would call the Keynote, is called the Final, being the note on which the melody ends.

Each Plagal Mode begins on the Dominant, four notes below the Final of its corresponding Authentic Mode (but ends on the same Final as its corresponding Authentic Mode).

The range of a Mode is usually an octave.

The difference between an Authentic Mode and its Plagal Mode is merely in the compass of the melody. (e.g. Authentic Mode: Final-Dominant-Final; Plagal Mode: Dominant-Final-Dominant).

In the 16th Century Henry of Glarus (Henricus Glareanus) wrote that there should be two more Authentic Modes, with Finals A (Aeolian Mode) and C (Ionian Mode), and their corresponding Plagal Modes. These Modes already existed from as early as the 11th Century in the secular music of the Troubadours, though the Church had used only the eight Modes of the Gregorian System.

The Ionian Mode (Final: C), or Major Mode, was known as the "wanton Mode".

Glareanus also (incorrectly) gave the Modes the names by which we know them today. The names of the Modes came from the names of the ancient peoples whose music they were believed to represent.

The Plagal Modes take the prefix Hypo (= under, or below) before the Mode's name.

A Mixed Mode extends over the range of both the Authentic and Plagal Modes
having the same Final.

Any Mode, as long as its order of tones and semitones is maintained, can be transposed to any pitch.

Click to see the Authentic and Plagal Modes, all with a Final of D

The Modes dominated European Music for eleven hundred years, from about the 5th Century to the 16th Century, and were still important, to a diminishing extent, for a considerable time after that. Quite a few modern composers have revived the use of the Modes in their music.

Much of the Folk Music of Europe, particularly Eastern Europe, is based on the Modes.

Many Modal Folk melodies still exist in Western Europe and the British Isles.

However, in the 18th Century, some arrangers and publishers "tidied up" or "civilized" Modal Folk tunes for "polite" Society by turning the Modes into Major or Minor Keys and making the rhythms regular. Many of the old tunes were lost or changed forever. (For example, there are at least two different published versions of the "Tiree Bridal Song" (Folksongs 3, page 57), one in the Mixolydian Mode, and the other made Major by sharpening the F; some publishers print "The Song of the Volga Boatmen" in a Minor Key with an extra flat as an accidental, instead of in its correct Phrygian Mode).

Many Folksong melodies collected in the British countryside have been found to be Modal. Those in either Major or Minor Keys are either of more recent composition, or are old tunes which have changed with the times, or are really in the Aeolian or Ionian Mode.