A Brief Tutorial on Musical Modes
by John Chambers

This is a relatively short description of musical modes. I'll assume that you understand the basics of ABC notation. If not, I have an introduction to ABC that you might want to read first. There are a number of longer documents on the subject of modes, which you might want to read. Here are some of them:
  • Jack Campin's Scales and Modes in Scottish Traditional Music
  • Jos Hindriks' Irish Modes tutorial.
    Let's start with a list of an octave of notes on a keyboard:
                  C ^C  D ^D  E  F ^F  G ^G  A ^A  B  c
    This is what we call the chromatic scale, and not much music uses it. Within this set of notes, there is a C major scale:
    chromatic     C ^C  D ^D  E  F ^F  G ^G  A ^A  B  c
    C major       C     D     E  F     G     A     B  c
    This subset of the notes has a somewhat irregular pattern. The distance between two notes in the chromatic scale is called a semitone, and we might also list the skips here in terms of the semitones:
    chromatic     C ^C  D ^D  E  F ^F  G ^G  A ^A  B  c
    C major       C     D     E  F     G     A     B  c
                     2     2    1   2     2     2    1
    This pattern, 2212221, is what makes a major scale, which has been the main sort of scale in Western music for the past couple centuries. Starting on C is significant here: C is called the tonal center or simply the tonic of this scale. The pattern of skips, 2212221, is called the mode.

    We can start a major scale on any note. If we start on D and use the same pattern of skips, we get:

    chromatic     C ^C  D ^D  E  F ^F  G ^G  A ^A  B  c ^c  d
    D major             D     E    ^F  G     A     B    ^c  d
                           2     2    1   2     2     2    1
    This is the same pattern of skips, so its the same mode, but it has D as the tonic rather than C. Similarly, we can start at ^D, though in this case it turns out better to rename the sharp notes as flats, and say we're starting at Eb:
    chromatic     C _D  D _E  E  F _G  G _A  A _B  B  c _d  d _e  e
    Eb major              _E     F     G _A    _B     c     d _e
                              2     2    1   2     2     2    1
    Now you know basically how a major mode (or scale) is constructed.
    Major may be the most important mode in Western music. (That's why it is called "major", by the way.) But it is not nearly the only mode that we use. The second important sort of scale is called minor (because it's of less importance in Western music than the major scale). Its basic pattern is shown most simply by the A minor scale:
    chromatic     A ^A  B  C ^C  D ^D  E  F ^F  G ^G  A
    A minor       A     B  C     D     E  F     G     A
                     2    1   2     2    1   2     2
    The 2122122 pattern of skips is called the minor scale or mode. We can apply this sequence of skips starting at C, and we get:
    chromatic     C ^C  D ^D  E  F ^F  G ^G  A ^A  B  c
    C minor       C     D _E     F     G _A    _B     c
                     2    1   2     2    1   2     2
    There is an interesting complication about the Western minor scale that should be described now, because it will come up again in other modes. The above scale is what is often called the natural minor scale, and is also called the Aeolian scale for obscure historical reasons. (Major is also called Ionian, but you don't need to know that.)

    The complication about the minor scale is that the two top notes are actually a bit ambiguous, and can be a semitone higher. Classical musicians talk about "ascending", "descending" and "harmonic" minor scales. These can be summarized as:

    chromatic     C ^C  D ^D  E  F ^F  G ^G  A ^A  B  c
    Cm ascending  C     D _E     F     G     A     B  c
                     2    1   2     2     2     2   1
    Cm descending C     D _E     F     G _A    _B     c
                     2    1   2     2    1   2     2
    Cm harmonic   C     D _E     F     G _A        B  c
                     2    1   2     2    1     3     1
    This looks complicated, but it isn't really. All it means is that musical passages mostly use the "ascending" notes when the tune is going upwards, and the "descending" notes when the tune is going downwards. And the "harmonic" scale? Well, all that means is that chordal harmonies mostly use these notes. Also, tunes that jump around in chords will usually use the "harmonic" notes.

    Note something new in the harmonic minor scale: There is a skip of three semitones between the 6th and 7th of the scale. We will see this again later. Minor-mode tunes in the western European traditions rarely have such skips in scale passages, but such scales are common in many other musical styles.

    The major and minor modes are the most common scales in most of the world, actually. But there are a great many other scales in use. Let's look at a couple of others that are very common in the traditional music of Ireland and Scotland, and are also heard a lot in the Balkans: the Dorian and Mixolydian modes.

    These funny-looking names have an obscure historical origin. In the Middle Ages, European musicians decided to name their scales after assorted ancient Greek regions. There is no real evidence that these modes have anything to do with the corresponding geographical areas, but the names have stuck.

    The Dorian mode is a lot like the minor mode:

    chromatic     A ^A  B  C ^C  D ^D  E  F ^F  G ^G  A
    A Dorian      A     B  C     D     E    ^F  G     A
                     2    1   2     2     2    1   2
    Can you spot the difference? It's the 6th of the scale. If you look back at the A minor scale shown above, you see that there's an F natural, but the A Dorian mode uses F sharp.

    Most of the time, if someone tells you that an Irish or Scottish tune is "in minor", it is actually Dorian. The terminology is used very loosely, and what "minor" means in this case is that the third of the scale is flat.

    Next, let's make one small change to the Dorian scale, by raising its third:

    chromatic     A ^A  B  C ^C  D ^D  E  F ^F  G ^G  A
    A Mixolydian  A     B    ^C  D     E    ^F  G     A
                     2     2    1   2     2    1   2
    This 22122122 pattern sounds a lot like a major scale, but the 7th of the scale is flat. You hear this scale a lot if you listen to Scottish or Irish bagpipe music, because that instrument is built so that a Mixolydian scale is "natural". This is done, in turn, because such scales are so common in the music. This produces a historical feedback loop that perpetuates the scale, which is how a lot of musical styles operate.

    Again, people often use loose terminology, and will say "major" for Mixolydian, because the third of the scale is sharp. We might put our four scales starting on A together:

    chromatic     A ^A  B  C ^C  D ^D  E  F ^F  G ^G  A  pattern
    A Aeolian     A     B  C     D     E  F     G     A  2122122 (minor)
    A Dorian      A     B  C     D     E    ^F  G     A  2122212 (minor but ...)
    A Mixolydian  A     B    ^C  D     E    ^F  G     A  2212212 (major but ...)
    A Ionian      A     B    ^C  D     E    ^F     ^G A  2212221 (major)
    Remember that Ionian = Major and Aeolian = Minor. Well, sort of, except that Dorian is also called Minor and Mixolydian is also called Major.
    ... to be continued ...