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background: I'm an amateur and self-learner pianist. What I know, I know from books or google, so I'll start with summarising what I already know - please don't hesitate to correct me if I have something wrong here.

I know that in western music there are different "modes" (= kinds of diatonic scales?). All that modes have a lot in common: they build the whole octave from intervals of 5 full tones and 2 semitones in some order. Various permutations of those intervals yield different modes, and the only constraint is that the two semitones must have 2 or 3 full tones between them.

Different such permutations (= different modes, = different kinds diatonic scales, synonymous?) have different Greek names: T-s-T-T-T-s-T = Dorian mode, T-T-s-T-T-s-T = Myxolydian mode, etc, etc. Cool.

Then:

  • Ionian mode, T-T-s-T-T-T-s, is also called "Major diatonic scale",
  • Aeolian mode, T-s-T-T-s-T-T, is also called "Minor diatonic scale".

Almost all songs I encouter are written around either of the above scales (in an arbitrary key). Why those two (and only those two, I suppose) got their own "mainstream" names and are used much more commonly than the remaining possibilities, and the rest are somehow "in the shadows"? I'm used to names such as "Toccata and Fugue in D minor", but I've never heard anyone entitling a piece "Waltz in C Dorian", for instance. Do composers avoid those scales or something? Doesn't the variety allow for more expresiveness? Is there any practical or historical reason for that phenomenon?

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Major and Minor are most common, but some composers don't state the mode, sometimes because the mode changes or the piece ends on a C - E - Eflat - G chord. The first movement of Joseph Jongen's Symphonie Concertante is an example of a piece written in Dorian mode. –  Mark Lutton Jun 7 '11 at 22:45
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3 Answers 3

up vote 10 down vote accepted

First some background if you weren't already aware: in any tonal key, say, C major, the first note of the scale, i.e. the root, is called tonic, the fifth is called the dominant, and the 7th is called the leading tone.

Quite a lot of common practice harmony exists due to the properties inherent in the harmonic series. If you're not familiar, it starts with an open 5th interval, and then above it is in the shape of a dominant 7th chord. (From the bottom going up starting on C: C, G, C, E, G, Bb.)

A major triad sounds the way it does because it fits into those first notes of the harmonic series. The flat 7th (in C major, Bb) actually used to be the norm, if we go back quite a ways in early music. The German musical system even today uses B to refer to what we call Bb, and H to refer to what we call B natural. In common practice, the dominant 7th is a chord that resolves to a tonic chord because chord tones only have to move by a half step . Think G-B-F (G dominant 7th) resolving to C,-C-E (C Major). These half steps are important: the B natural in C major is just a half step below the tonic, and we call it the leading tone.

Another part of the answer has to do with how major and minor are generally defined by triads, not by entire scales. If you take a major scale and start planing triads up and down diatonically, you're going to get mostly major and minor chords, plus one diminished triad based on the leading tone. The fact that lots of music was written in just one key meant that major and minor were the primary chords available for use since you could make them both with the notes present in the major scale.

Also, traditional minor breaks a lot of the rules of diatonic modality to fit with common practice. The flat 3rd stays the same, but if you look at a melodic minor scale, the natural 6ths and 7ths are used in addition to the flatted ones.

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The German musical system (H and B) is actually what I've been consistently taught back in primary school here in Poland :) Thanks for explanation for the rationale behind it! –  Kos May 26 '11 at 19:42
    
The fourth paragraph indeed explains to me why for instance Phrygian mode (white keys from E) could be troublesome- I couldn't have the dominant chord with only the tones from the scale (and Locrian would not even allow me to have the tonic chord this way), which in order would hurt the harmonisation (is that the point?). But Dorian (D) still has I, IV and Vth chords in the scale... Is there any particular advantage of having the dimnished triad placed on the leading tone? –  Kos May 26 '11 at 19:48
    
@Kos The way music ended up being written, composers often resolved the diminished triad to a major chord up half a step. On the leading tone it goes right back to I. You should probably also realize that the equal temperament system of pitches we use now is not what was being used when all of this was being developed. Most of it holds true today, but harmony sounded a lot different once upon a time. –  NReilingh May 26 '11 at 20:01
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The Ionian mode IS the most stable mode. NReilingh explains why in his answer. However, the fact remains that MUCH of music is actually written in modes without appearing that way.

The Ionian mode is usually the basis for the key signature of the song's music. For example, the "key" of C has no sharps and no flats because playing from C to C with no sharps/flats would produce an Ionian scale.

However, there are two ways that a piece can "slip" into another mode:

  • The tonic changes: Say you start out in the key of C. Somewhere in the middle of the piece, the tonal center, the "tonic note" changes (subtly) to F, for a short time. For that short passage, you could say that the piece temporarily shifts to F Lydian. The only way to identify this is with listening and analysis, and even then, its open to interpretation whether or not you want to say the piece "shifts"

  • A consistent accidental appears: Starting in the key of C again. For a short time in the passage, be it a few measures or a few lines, you might have a recurring accidental. In C, an F# might appear. If you rule out that it is not a modulation or leading to a modulation (i.e. C remains the tonic), then you are effectively in C lydian. Again, you have to determine by listening and analysis what note remains the tonic note? It's not always the name of the key signature.

The point of the above (I hope it was coherent), is to say that the "other" modes are not used "officially." Ionian (and slightly lesser so, Aeolian) retain a special status for good reason, most pieces start and end up there, so it makes sense to name the write the key after the Ionian scale of the tonic.

Most importantly: the tonic note does not always have to be the name of the key signature. This is when modes are used

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I personally use scales more like a concept of theoretically explaining and understanding music after it was written. The composers/artists write music that sound good and by coincidence you find out that certain structures in a piece have certain effects and are therefore repeated in many other pieces.

For a non-experienced listener, music that uses uncommon scales is usually more difficult to hear/understand/appreciate than music with plain major or minor scales, that's just what he's used to. In jazz music for example, all kinds of different scales are used (see Wikipedia article on modal jazz for example), but the music is difficult to appreciate for many people.

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Hmm, I do agree with this but I think you need to address why major/minor scales are so popular for it to be a complete answer. –  Matthew Read May 26 '11 at 18:29
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You are suggesting that the composers don't pay attention to the scales and all the theory when composing - are you sure of that? My guess would be that there are both "ear composers" and "math composers", and that the average composer would be somewhere in between these two approaches... But I don't know :) –  Kos May 26 '11 at 19:58
    
I haven't heard of modal jazz before, thanks! That really seems interesting... Even more than jazz itself :D –  Kos May 26 '11 at 19:59
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The problem with this is that the modal system predates the tonal system. So, at some point in history, modal systems were considered more natural than tonal ones. When you go to other societies where music developed in a different way from the West, you'll see that they also lean towards modal systems. The key I think is the development in the West of polyphony. Polyphony is the main reason why the tonal system imposed itself in the West, although I don't know all the details. –  Raskolnikov May 26 '11 at 21:27
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