the progression you are referring to is I-IV-bIII-bVI, where are all these chords are “power chords” i.e. dyads comprised of a root and fifth.
it is an awesome progression but violates absolutely nothing in music theory.
we can find examples of similar progressions across jazz and classical music, most of which was composed by folks who have mastered western tonal harmony.
how would you use this approach in practical application?
i haven’t met many working musicians who had much difficulty learning the relationships between different keys, how they connect to the circle of fifths (fourths), and key signatures.
i get that it can seem overwhelming and non-intuitive, but it’s really not that complicated once you spend time playing and practicing music that illuminates these relationships (like playing ii-V-I progressions in every key, going around the circle of fifths). very little memorization involved; moreso muscle memory and an accumulation of applied theory in context.
most of the musicians i know are jazz players, where being able to play in any key is a critical aspect of mastering the genre. all the classical musicians i know are professional orchestral musicians, and they don’t seem to have any difficulty either.
the higher you go in frequency, the physical distance between each interval on the fretboard becomes smaller. so if a +/- 5 cent adjustment is 0.5 mm at the first “fret” after the nut, it will be something like be 0.1 mm when you are at the 7th “fret” location (i.e where the interval of perfect fifth, relative to the open string, is played).
the sandman (*) intervals aren’t coming from microtonal tuning… it’s dynamically modulated detuning in equal temperament, just as you say. it’s an extremely common type of modulation, especially if there are synthesizers involved.
so slightly that it can be on the range of 0-5 cents, provided the instrument is sufficiently constructed and the player is sufficiently skilled.
this is why a guitar using equal temperment can play consistently in-tune with itself as well as with other instruments tuned in the same system. it’s not about perfection according to some abstract mathematical model.
yes, I am aware that conventional guitars have fundamental issues with intonation in equal temperament systems.
this has not prevented it from being a versatile instrument that is quite capable of being played “enough” in tune with ensembles of other instruments, such that the vast majority of people hear zero problems.
how is what we hear in music less relevant than whether or not a given instrument is not perfectly in tune, mathematically speaking, if that variation in tuning is imperceptible to human hearing?
