When analyzing the notes in a scale or key, some scales are very simple and some are a little more complicated.
Take the key of C, for example and its derived modes. The C major scale has no sharps or flats and its 7 notes are C – D – E – F – G – B.
All the modes derived from the key of C are also simple. For example, D dorian, the 2nd mode of the C major scale contains D – E – F – G – A – B – C
Then there are some keys that are bit more complicated. Take A sharp major scale, for example. The notes in A# major are A# – B# – Cx – D# – E# – Fx – Gx.
The A# major scale contains 3 double sharps (x). This by itself is not too difficult to understand. Occasionally, being familiar with A# major is useful for a number of reasons. For example the chord A#maj can often be found in different songs so can be useful to at least understand the key of A# major even though it is much easier to use its enharmonically equivalent key, Bb major scale.
If we are studying the key of these slightly unusual keys (such as A#) then we should also explore the modes that are derived from these keys. The 2nd note of the A# major scale is B#, therefor we could also explore the notes in B# dorian (B# – Cx – D# – E# – Fx – Gx – A#).
Of course, B# dorian and A# major contain the same amount of sharps and double sharps. B# dorian itself is fine to work with, much like A# major.
The Problem With Some Modes
The problem of the ‘impractical mode’ comes in when we look at the 3rd mode of A# (again, for example). The 3rd note of A# major is Cx. Therefor, theoretically the 3rd mode of A# major is C double sharp phrygian.
While playing scales and modes that contain double sharps and double flats is ok (maybe a little annoying, but ok) playing modes that have a double sharp or double flat as their root note is impractical for a number of reasons. Firstly, it’s highly unlikely that you would ever need to use Cx phrygian in a real life musical context. Secondly, Even though we could theoretically explore and play the mode Cx phrygian, the study would not be complete without comparing it to Cx major. In looking at Cx major, we would find that it contains the notes
Cx – Dx – Ex – Fx – Gx – Ax – Bx
You can see how things now are starting to look a bit ridiculous. To take it even further, if we were then to explore the modes of Cx major, we might then want to explore the modes of Cx major. In doing so, we might look at another impractical mode such as Dx Dorian and from there we would have to analyize Dx Major. In theory, the key of D double sharp major contains 2 triple sharps! You could play this game forever and it becomes very silly very quickly.
Any mode that has its root note as a double sharp or double flat, is now deemed impractical.
The Problem with Ab Locrian
There is one more way that a mode can be deemed impractical:
Let’s look at a mode (in theory) by the name of Ab locrian. This might seem like a logical mode to want to explore. For example, the locrian mode can be used over minor chords and the Ab minor chord is frequently used. Also, the Ab major scale contains only 4 flats, which isn’t too complicated right?
The problem with Ab locrian is that it is technically derived from the impractical key of B double flat. It is not derived from A major because that would produce G# locrian, not Ab locrian.
In theory, the notes in B double flat major are:
Bbb – Cb – Db – Ebb – Fb – Gb – Ab
As Ab is the 7th note and the locrian mode is built upon the 7th note of the major scale, Ab locrian is derived from Bbb. This is impractical. Again, it is fine to play a scale or mode that contains double sharps or double flats, but the exploration of keys that have double flats or double sharps as their root is a frivolous one.
Any mode that is derived from a key that has a double sharp or double flat as its root note, is now deemed impractical.
The name ‘impractical mode’ is really just a term that I have come up with for the study of guitar modes explained. In this study, we may occasionally mention the notes that are in a given impractical mode just for theoretical clarification, but we will not explore the mode in great detail. It is really a matter of where you draw the line. There may be some (sad) people who believe that exploring the key of Dbbbb is worth doing. There may also be people who think that the key of A# is not practical because it contains double sharps and can be replaced (sort of) with the key of Bb. Again, there is no real right or wrong here, it is just a matter of where you draw the line.
The 2 rules that I have mentioned above, is where I have chosen to draw the line.
If you don’t already have a solid understanding of how guitar modes work, everything that has been mentioned here will probably confuse you. If that is the case, don’t worry too much about it. When you see an impractical mode mentioned, ignore it and move on. Once you understand modes inside out, everything in this post will make a lot more sense.