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When I search for the definition of key and mode, I see two arguments:

  1. When we talk about key, we only talk about major and minor. Letter name plus major and letter name plus minor are the only keys possible, D dorian is not in the key of D, it's in the key of C major. D dorian is the second mode of C major key. Modes are "children" of keys. And when we talk about pop songs and chord progressions we usually ignore the minor key, we treat it as its relative major key, so if the song is A minor, with the first chord being the Am chord, we write it as vi chord in C major.
  2. Key only defines the "home" note, a.k.a tonal center, or tonic. If the melody arrives home at D, it's in the key of D. But the key didn't define notes other than the home, mode then defines other notes based on intervals derived from the home note. So D dorian means it's in the key of D, and using dorian scale with D being the tonic. Major is also just another mode, in the case of C major, C is the key and major is the mode. so letter name is the "key" and "scale name" is the mode. Keys and modes are parallel, not parental.

They are contradicting each other. Where do each of them come from? I have seen on both sides the support of different university theory professionals. Which one is correct? Or is there still no consensus? Which side has the greater number of supporters?

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    "And when we talk about pop songs and chord progressions we usually ignore the minor key, we treat it as it's relative major key." This is false, so your first step is to find a better source of information.
    – Aaron
    Commented Mar 29 at 5:47
  • The second case, I don't quite understand what you mean with "So D dorian means it's in the key of D, and using dorian scale with D being the tonic. ". Which key are you using, are you using the dorian key or major key or minor key or ...
    – Emil
    Commented Mar 29 at 6:45
  • "This is false, so your first step is to find a better source of information" It's a thing in the Chinese pop music theory community, I see people calling it "一元制", I don't know how to translate it to English, maybe "single-tonic-notation"? The point is, chord progressions like Am F G Am, instead of i VI VII i, it's written as vi IV V vi as if it's C major. And I also see people mentioning the name of "Walter Hamor Piston" during some discussions.
    – Eary Chow
    Commented Mar 29 at 6:45
  • "The second case, I don't quite understand what you mean " I mean, the “D” in D dorian defines the home note, the that's what a key is. Then dorian, or major, or minor, or lydian etc. are all modes. So key is only the letter name, while the thing afterwards is mode.
    – Eary Chow
    Commented Mar 29 at 6:46
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    Okay then I misunderstood you. As an engineer I don't see the point in trying to choose the correct between these, in the end its just a tonic and a procedure to get the other pitches and giving them names. You could think of many ways to do that. I think you only need to care about this if you meet a very obstinate theoretician.
    – Emil
    Commented Mar 29 at 6:56

4 Answers 4

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Where do each of them come from?

Key and modality are complex topics. Beginning theory students are not in a position to appreciate the complexities and inconsistencies. Even if they were, it is difficult to formulate a concise description that is also consistent with the complexities and exceptions. People have therefore developed different ways of explaining key and mode.

Which one is correct?

This depends on your definition of "correct." If you mean "which one of these describes the way practicing musicians use the concept of key to communicate," then neither is correct.

Or is there still no consensus?

Probably not. Some people certainly think of the major mode as the "parent" mode, but even a lot of those people -- including some users on this site -- recognize that the choice of "parent mode" is fairly arbitrary.

Which side has the greater number of supporters?

I doubt anyone knows. In any event, because of the complexity of tonality and modality, I 2xpect that even the most zealous supporters of each theory will agree more with each other than disagree once they start to discuss nuances and exceptions.

As a bonus, here are some incorrect statements in the summaries of the two approaches. You may want to check whether you've misinterpreted any of the sources you've seen or whether they are simply incorrect:

  • When we talk about key, we only talk about major and minor: I don't believe I've encountered anyone who refuses to acknowledge the modes.

  • Letter name plus major and letter name plus minor are the only keys possible, D dorian is not in the key of D, it's in the key of C major: if this were really true, we would not need to have the phrase or the concept of "D dorian."

  • Modes are "children" of keys: Historically, it is the other way around. Modes existed for centuries before keys. Modes as children of keys is a theoretical construct that may be useful for certain purposes, but it does not have any claim to being fundamental truth.

  • And when we talk about pop songs and chord progressions we usually ignore the minor key, we treat it as it's relative major key, so if the song is A minor, with the first chord being the Am chord, we write it as vi chord in C major: I've never encountered anyone who analyzes music this way.

  • D dorian means it's in the key of D, and using dorian scale with D being the tonic: songs rarely use only the seven pitch classes of one mode. For example, Haydn's famous Surprise Symphony tune, a very straightforward melody in C major, uses C, D, D♯, E, F, F♯, G, A, B, and C.

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  • Quite a lot of claims that songs have a "vi-IV-I-V" chord progression ignore that several of those songs are in minor keys, such as F-777's "Dance of the Violins". Only if those songs are treated as being in the relative major instead do those songs have vi-IV-I-V chord progressions (instead of i-VI-III-VII like they really do).
    – Dekkadeci
    Commented Mar 29 at 7:51
  • @Dekkadeci I suppose part of the reason might be it's ambiguous, if a song uses vi-IV-I-V progression of C major through out, depending on the melody and other aspects you might still hear part of the song as in C major and some parts in A minor, etc. However, it would be confusing to switch to the "correct" spelling when it's in C major (vs. A minor), especially when the transition is not clear cut, so people treat it like it's the same key and the convention is just to use C major.
    – Divide1918
    Commented Mar 29 at 8:39
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The defining factor of major or minor is ^3. In major, the interval between it and the root is 4 semitones, in minor, 3 semitones. Thus in key D, D major is defined by the presence of F♯, in key D minor, the presence of F♮.From the parent key C.

Often, saying simply 'key D' will refer to the major key - if it's Dm, then the 'minor' tag needs adding.

So far so good. With modes, there are still majors and minors, using the same guidelines - that 3rd note. The difference though is that each mode has a specific title, or name. Those in themselves are indicative of maj/min. When one refers to D Dorian, it's implicit that it's a minor mode - it never was or would be otherwise. Similarly with E Phrygian - m3 interval, therefore, a minor mode.

Then follows two major modes, Lydian and Mixolydian, both with the natural 3rd. And the 6th mode, Aeolian (A in my example), has m3 between root and ^3. Hardly a need to involve Locrian at this point!

So there's little or no mystery here.

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Note that strictly speaking the concept of key and mode stem from two very different styles and practices of music. In modal music the mode would determine a few principal notes, such as the finalis and the cofinalis. In tonal music the key gives a tonal center and a set of dominant pitches which we’d call a scale.

For a long time these scales were usually either the major scale or an extended minor scale, occasionally modulo some chromatic alterations. Then with modulations, borrowing and obscuring the tonal center these scales lost a lot in importance over the course of 19th and early 20th century.

Finally there was music with a tonal center, but without significant restriction within the chromatic (and even subchromatic) scale. This made the whole concept of major or minor key hard to justify.

Also we get cases of usually melodies borrowing structure from certain modes, which gives a reading of modes as scales (here one should differentiate between a mode and a modal scale). This gets especially popular in Jazz music.

But this then leads to different approaches and thus different definitions of key and tonality. For heavily extended tonality if makes sense to say: The key only gives the (primary) tonal center, and then you might or might not restrict yourself to some scales. For less extended tonality if might make sense to define the key as the combination of a root with one specific scale.

If you are a pop musician chances are high that your music is not too extended in terms of tonality, so it seems like this would be a reasonable reading. But in pop music it tends to be quite common that the tonal center is not that important, and you are supposed to play a song in different transpositions. This then leads to a system where you do not think in specific chords, but chords relative to the current root. Here as you said minor keys are usually notated as relative major keys, as this allows the musician to develop these skills much more easily (the 6 to C♯ will always be the same, no matter what the tonality of the song is).

So in this sense a key is actually the tonality of a major scale that provides as many of the required notes as possible, so it does not give the root, but only the set of primary notes used.

So you see that these concepts can and should be used and defined differently in different schools and styles of music, simply because these will have different requirements. The theory serves to understand the practice, not the other way round. And this means that if practices differ a lot it does not make sense to enforce a theory that works for one to the other. And often these definitions will be fuzzy, and a modern temporary composer will both understand a piece being in C, as well as a piece being in C major. A pop musician will understand a piece being in A minor as well as a piece being in C major.

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The concept of key is more elaborate than just the home note. A key (in current practice) is a set of melodic and harmonic relations to a tonic note. Major and minor keys differ in both their melodic and harmonic conventions. There is much more but it would take at least three books to outline the concepts.

Among the harmonic conventions are: the tonic chord (based on the tonic), the dominant chord, and the subdominant chord. Other chords have conventional relations. In the key of C major, the tonic chord is C-E-G, the dominant is G-B-D, and the subdominant is F-A-C. The tonic is signaled through certain cadences; classically, the cadence F-A-D-C is followed by G-C-E-G, G-B-D-G (or G-B-D-F), and C-E-G-C chords (with some adjustments to avoid parallel fifths or octaves. Likewise, some non-diatonic chords "belong" to a key. In C, the chords F-Ab-Db-F, Ab-C-Eb-F#, Ab-C-D-F#, and Ab-C-F#, have special meaning. When the tonic changes, all these relations move to those of the new tonic.

In Guido's day, "mode" referred to the melodic classification of chant melodies. The finalis was the final note of the chant while the tenor (also called the reciting tone) was the note used most often. Each of the eight modes had a different finalis, tenor, and range of notes. The concepts of finalis and tenor morphed into tonic and dominant (though not with the same notes) in key-oriented theory.

I'm not that knowledgeable about the modern use of mode. Different modes seem to refer to permutations of diatonic notes of a scale. The harmonic terms dominant and subdominant are not as well defined.

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