Mutated 12-bar blues

David Bennett has uploaded a few YouTube videos discussing common chord sequences, eg  i bIII bVII IV (Em G D A in the key of E minor, although this is really E Dorian because of the C# in the sub-dominant A chord) or the 'doo wop changes': I vi IV V (E C#m A B). He makes the interesting point that modern songs seemed to be composed entirely of 4 bar loops that don't really resolve, where 'resolving' means a complete cadence, V -> I, as in the doo wop changes. The first sequence (that DB terms the 'Wonderwall' sequence) can also be referred to as a triple plagal cadence in which the four chord resolves to the one: G is the fourth/sub-dominant of D, D is the sub-dominant of A and A is the sub-dominant of E(m). The plagal cadence is a weaker form of cadence and doesn't really resolve in the same way that the dominant to the tonic does.

For years, I've been trying to avoid full cadences (trying frequently to use imperfect or deceptive cadences, if I'm writing a functional sequence) and also songs that contain endlessly repeating four bar sequences. As it happens, the song that I wrote last year, "My father's eyes" is basically an endlessly repeating four bar sequence, but not one that DB mentioned (i v bVI IV - Dm Am Bb G): this is displaying modal mixture, as no key can have as diatonic chords Bb and G (because of the Bb/B dichotomy).

The YT video that I linked to ends with a discussion of the twelve bar blues, a form that is very well known, at least in the 20th century, and doesn't match those endless four bar sequences. It normally consists of four bars of the tonic, two bars of the subdominant, two bars of the tonic then two bars of the dominant and two bars of the tonic, although the last four bars can be different. One could write this as I x 4, IV x 2, I x 2, V x 2, I x 2, or for variation, I x 4, IV x 2, I x 2, V, IV, I, V. 

A few years ago, I realised that the song "Riders on the storm" by The Doors is actually a 12-bar blues, albeit in a mutated form. First of all, it's in a minor key and due to that, there are chord substitutions. I want to compare this to a standard major 12 bar: if "Riders" is in Em, then a 12-bar in E (major) would be E x 4, A x 4, E x 2, B A E B (using the turnaround form for the final four bars). "Riders" could be written as Em x 4, Am x 2, Em x 2, D C Em x 2 (or Em D). The first eight bars have the same structure, with minor chords replacing the major chords. D replaces B and can be seen as a flattened form of the same chord: B Major contains the notes B D# F#, and if it were B7, the notes would be B D# F# A. If we flatten the third, we get B D F# A (Bm7) than can also be seen as D6. And if we drop the B altogether, the notes are now D F# A - D Major! The same correspondence exists between C and A, where C can be seen as a flattened version of a rootless A chord.

So "Riders" is a 12-bar blues, not that anyone would ever consider it so. 

Bonus points: in the middle of "21st century schizoid man" by King Crimson, there is also a 12-bar blues hiding in full sight, starting at 4:40 in the original version (until about 5:20). One listens to the song and is so amazed by the unison playing and the extremeness of this section, that the fact that this is a 12-bar completely escapes one!