A Non Structure
After all the different musical structures we've looked at here's a break from that - a non-structure!
Indian classical music has incredibly involved scales and 'ragas' which correspond to modes/moods in western music - however Indian music uses no chord changes at all.
Everything is played to a backing ' drone ' of the notes C and G [ a C5 chord ]
With such a ' 5 chord ' backing drone any of the scales that root on C could be played [ Indian classical music has lots of them ] plus modes of other scales.
Jimi Hendrix used this technique by setting up a ' drone ' either by the bass-guitar playing root and fifth or using an open guitar string and then soloing through first one scale and then another - a great deal of freedom of choice of notes is possible because there are no chord changes to follow - the ' tonal centre ' remains constant.
Check out the Byrds classic ' Eight Miles High ' - the intro and the guitar solo both use this ' drone ' style backing - whereas the vocal parts of the song have standard chord changes.
The Beatles tune ' Tomorrow Never Knows ' is another classic example of this style. [ This probably came from the influence of Indian music on 60's psychedelia. ]
Another , slightly different , aspect of this style is to be found in some forms of Blues music - a lot of early blues had no chord changes - possibly the African antecedents of the music were monochordal and relied more on rhythm for their effect.
Check out Howlin' Wolf's ' Smokestack Lightning ' for a great song with just one chord throughout.
' Spoonful ' by the same artist or by Cream.
' You Need Love ' by Muddy Waters which was transformed into ' Whole Lotta Love ' by Led Zeppelin .
' East-West ' by the Butterfield Blues Band and lots of great tunes by Bo Diddley share this minimalist approach.
Hypnotic repetition for effect can also be seen as an aspect of this monochordal style.
Many modern Dance/Trance music styles follow this pattern.
These are all perfectly valid musical techniques and so although this isn't a structure as such , it is a part of the overall picture.
Tune your bottom E string down to D [ dropped D tuning ] - this makes your bottom pair of strings a D5 chord.
Then try to set up a drone with these two strings while playing around with D scales on the other four strings.
e.g. D Major scale - different D minor scales - D Pentatonic - D based modes of other scales ( G major from D to D for example ) etc.