Why are certain scales considered unstable?

Question

The Locrian Mode is considered unstable - my question is what in it makes it unstable? What makes a scale stable (have better relation between the keys)?

Answer 1

Perhaps the most important reason it sounds unstable is because the tonic chord is actually a diminished triad. It's the ONLY one of the "white key" church modes that has that structure. If you think about it, locrian mode starting on B is spelled B-C-D-E-F-G-A-B. Using only the notes that are part of that mode, and building a triad from degrees 1, 3, 5 results in B-D-F which is a B-diminished triad.

Our western ears are attuned to wanting such a diminished triad to resolve to a major or minor triad, and to hearing it tend to resolve, in this instance to a C-major or C-minor triad - e.g. C-E-G or C-Eb-G (I in C-major; i in C-minor), or else, perhaps to an A-minor triad A-C-E (i in a-minor), because we tend to hear diminished triads has having either a leading tone - viio (major or minor key) or supertonic - iio (minor key only) function.

Answer 2

Hi there, I think I can help answer your question.

The word "unstable" refers to use of tension and release within a melody or chord progression. In a major scale/key, lets say C major, it is easier to create tension and release because our C chord gives us a "release" when we return to it. For example, our ii-V-I progression or Dmin-G-C has a certain sound to it that builds tension with Dmin to G and then releases that tension when we return to our C chord.

With our Locrian mode, this concept is more difficult to execute as the intervals and chord progressions that are diatonic to our Locrian have such a specific and foreign sound to most people that it is considered "unstable" as there is no one chord or note that is able to completely release the tension that is build from a given Locrain progression.

With all of that said, the Locrian mode has its place and it can be really fun and inspiring to try composing something with the Locrain mode, but I think once you try you will see why the word "unstable" applies to it.

Hope that helps!

-Nic