Why Locrian #2?

Question

A scale built off the 6th degree of the jazz melodic minor (C-D-Eb-F-G-A-B-C) is spelled like this:

A-B-C-D-Eb-F-G

Paraphrasing Mark Levine's Jazz Theory Book, Chapter 3 - Modes of the Melodic Minor - The Half-Diminished Chord:

This mode is often called the Locrian #2, because the only difference between this scale and the Locrian mode of C Major is the M2. With A as the root, it's Bb in Locrian, vs B in Locrian #2:

Locrian = A-Bb-C-D-Eb-F-G
Locrian #2 = A-B-C-D-Eb-F-G

Question: Locrian #2 is simply the natural minor scale (Aeolian mode) with a diminished 5th (Eb vs E in our example) - its key signature could be written as one flat - Eb.

If so, why refer to this scale as Locrian #2, instead of Natural Minor b5, or perhaps just Minor b5 or Diminished Minor - arguably easier to remember and spell than Locrian #2.

Answer 1

This scale is typically played on a Amin75b (half diminished chord, second degree in G minor). A natural scale for this chord would be A locrian, but the 9b is quite harsh and should be "avoided".

A simple trick that jazzmen use when faced with an "avoid" note is to alterate it (examples : 1) C major on a Cmaj7 chord, F is an avoid note => change to F# => C lydian, 2) G mixolydian on G7, C is an avoid note => change to C# => G lydian-dominant).

Here, using this "fix", we obtain the locrian scale with a natural 9. This scale is sometimes known as "half-diminished scale".

At the end, name it as you wish (I prefer "half-diminished", avoiding your question completely), and most importantly, remember it as you wish (I remember it as the 6th mode of C melodic minor and I think "C melodic minor on Amin7b5" , not "locrian natural 2") but I think it is more natural to see it as a "regularized" locrian scale in this respect.

Answer 2

One possible reason is the inclusion of the b5.

In tonal music, scale-degree 5 is so vitally important as the basis of the dominant chord; it's of course this dominant chord that creates the ultimate tension that leads everything back to tonic. When a musical environment lacks this perfect fifth above tonic, the tonality it creates (or fails to create) is a staggeringly different one than one usually thinks of when s/he thinks of "tonal music."

(It's partially because of this reason that Locrian is often the "forgotten" mode---because it's the only one that has b5 instead of the perfect fifth.)

So it might come across as odd to call a scale "Natural Minor b5" (even if it technically is) because "natural minor" has such a clear connection to classical tonality (which demands that perfect fifth). With such a clear connection, it's almost an oxymoron to replace 5 with b5. It's a bit like a gym offering you a free extra large pizza when you're done with your workout; it undoes so much of your hard work!

In contrast, one of the most defining characteristics of Locrian is this b5. So when a collection includes this b5, perhaps it's best to list it as an adjusted Locrian scale (which has very little connection with classical tonality) instead of an adjusted natural minor scale (which does).

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There's another possible reason: the scale in question is a rotation, meaning that it's a scale collection that begins on a scale degree other than the tonic of a parent scale (here, C jazz minor). We're fine with the notion of a Locrian mode as being a rotation (it's the seventh mode of a major scale), but we tend to think of natural minor less as a rotation and more as a bona-fide scale based on its own tonic. (It's Aeolian, so it is a rotation, but there's also an entire tonality based off of it, so this is a different case.) This may be another reason; since this scale is a rotation, let's base it off of another rotation, not off of a more familiar tonal scale.

Answer 3

First, I'll say that I agree wholeheartedly with the answers provided by Richard and Alexadre C. It wouldn't make sense to look at a melodic minor scale and call it a "major ♭3 scale." The flat third is a defining characteristic of minor scales. The perfect fifth is, too, as Richard explains. Flatting the fifth takes the sound away from minor (to my ear), and I think the reason has everything to do with function. For example, in a minor iiø-V7alt-i progression, the ♭5 (the E♭ in an Aø7 chord) resolves down a half step to a D--the root of the V7alt chord. Additionally, the ♭5 (the E♭) also becomes the ♭9 over the V7alt chord, and the ♭9 is one of the defining notes in the altered 7th chord. The ♭5 thus is a distinguishing feature of the Locrian scale, and this function overtop a iiø chord is distinct from the function of a minor scale overtop a minor i chord. Further to the point about function, playing a ♭5 over a minor i chord sounds like a blues note and wants to resolve to the natural 5th. The Aeolian ♭5 scale wouldn't permit this resolution which is so foundational to blues music/licks.

There may be a guiding principle at work here: given two different naming choices (Locrian ♯2 vs. Aeolian ♭5), select the name which modifies a passing tone. The second is much more a passing tone than the fifth. Perhaps this rule guides one to choose the name which most closely preserves the scale's correct function.

Answer 4

It should more correctly be called Locrian ♮2 (natural 2), because there's no sharp involved. The reason why it is a natural 2 is that the structure of Locrian is

1 b2 b3 4 b5 b6 b7

So a major second (or ninth) from the root can be produced by a natural, resulting in the scale

1 2 b3 4 b5 b6 b7

which is the 6th mode of melodic minor.

The rest is just convention. It could correctly be called aeolian b5, it's just that almost everybody calls it locrian ♮2 (or, less accurately, locrian #2). This comes simply from the fact that that scale is used in contexts where locrian could also be used (over a half-diminished chord). The scale is of course an aeolian scale with a b5, but it is normally not used to replace the standard aeolian scale because the b5 changes the character of the underlying chord (from m7 to m7(b5)).

For all the modes of the melodic minor scale there are potentially two names, simply because melodic minor can be viewed as either dorian with a natural 7, or as major with a flat 3. Going through all the modes in that way you get the following theoretical possibilities based on the names of the modes of the major scale, some of which are more intuitively pleasing than others:

2nd mode: dorian b2     | phrygian ♮6
3rd mode: phrygian b1   | lydian #5
4th mode: lydian b7     | mixolydian #4 (#11)
5th mode: mixolydian b6 | aeolian ♮3
6th mode: aeolian b5    | locrian ♮2
7th mode: locrian b4    | ionian #1