We have the ancient Greeks to thank for the modern seven modes of music, although some of their original modes have been replaced over time. Greek musicians may have been the first to understand the intimate relationship of mathematics and music theory, leading to what we understand as the major and minor scales. A traditional scale may be divided into 8 notes, but the intervals between those notes are not always equal. The Greeks developed scales which began and ended on each note of the original major scale (Ionian) we still hear today. The Greeks named the different scales after cities which reflected the mood of the seven modes of music. Modern music theorists now call these modes Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Locrian.
Understanding the seven modes of music requires a basic understanding of intervals and musical notation. We can define a musical scale with 8 letters (A-G), so that a "C" scale becomes C D E F G A B C with no flattened or raised notes. But another way to look at this scale is through intervals. This is the musical theory behind the familiar Do Re Mi Fa So La Ti Do exercises in choir or early band practice. Each of those syllables represents an interval in a major (Ionian) scale. These intervals are not evenly spaced, thus giving each one a specific relationship to each other. "Do" sounds very finished and solid, for instance. "Ti", by comparison, sounds very unfinished and unresolved. It is called a leading tone, which means it wants to lead the melody back to the final note "Do". It's the relationship between these intervals which give the modes their musical interest.
In the Ionian mode, the intervals are divided into a very familiar pattern of whole and half steps. Most of us would instantly recognize the Ionian mode when played on a piano. The rest of the seven modes of music retain this familiar pattern, but start on different notes. Here's a quick breakdown on each mode and their relationship to the original Ionian intervals:
Ionian Mode (W-W-H-W-W-W-H) In this definition, W stands for 'Whole Step' and H stands for 'Half Step'. The Ionian mode defines the familiar major scale pattern we hear as do, re, mi, fa, so, la, ti, do. The half step between Ti and Do gives the scale some tension and release. The majority of popular songs are written in the Ionian mode.
Dorian Mode (W-H-W-W-W-H-W) Dorian mode is most commonly heard in Celtic music and early American folk songs derived from Irish melodies. Songs written in Dorian mode sound a little melancholy because the final note (re) doesn't quite resolve itself. The song may be over, but the singer is still unsettled.
Phrygian Mode (H-W-W-W-H-W-W) Modern composers and guitarists commonly use Phrygian mode because it works well with the Ionian. Guitarists use modal music to create interesting solo lines which can be played against melodies in other modes. Composers often find the Phrygian mode to be as useful as the traditional minor (Aeolian) scale, but without the inherent sadness.
Lydian Mode (W-W-W-H-W-W-H) Lydian mode is the complete opposite of the Ionian, so it feels as solid as a major scale but the intervals are surprising and unexpected. This is a popular mode among jazz musicians who enjoy using a mixture of major and minor chord progression in inventive ways.
Mixolydian Mode (W-W-H-W-W-H-W) Mixolydian is similar to Lydian in the sense of a major scale feel with minor intervals. Mixolydian mode is another popular scale for solo musicians looking for a counterpoint to the Ionian key of the song.
Aeolian Mode (W-H-W-W-H-W-W) Aeolian mode is still in vogue today, although we tend to refer to it as the minor key. The intervals of Aeolian mode create the same feel as many modern blues songs. Songs composed in Aeolian mode have a strong sense of sadness. The final note of an Aeolian scale feels resolved in a completely different sense than the Ionian. If the Dorian mode reflects melancholy, the Aeolian reflects despair.
Locrian Mode (H-W-W-H-W-W-W) Locrian mode is considered to be so unstable and unsatisfying that most composers consider it unworkable. There are few songs written in the Locrian mode, which has lead some music experts to label it a 'theoretical' mode. It exists because all seven notes of the Ionian scale could form modes in a mathematical sense, but the relationship between intervals in the Locrian mode is simply not that interesting musically.
To remember the seven modes of music, many musicians use the following memory aid: "I Do F(ph)ollow Lonely Men And Laugh."