Harmonic Relativity
Harmonic relativity is a music theory framework that places emphasis on the relationships between subsequent chords, rather than the relationships of those individual chords back to a key center.
Formalism
The concept of a key center is discarded entirely. Rather than labeling chords relative to a tonic (ii°, V7, etc.), only the relationships between chords are labeled. Each chord relationship (CR) is defined by 3 parameters: the qualities of the first and second chords, and the distance between their roots.
For example, the CR labeled M-II-m refers to a major chord, followed by a minor chord a major 2nd above it. Some chord sequences that fits this CR would be C major followed by D minor or A♭ major followed by B♭ minor.
For a more complex example, the CR labeled ø7-vii-7♭9 refers to a half-diminished 7th chord, followed by a dominant ♭9 chord a minor seventh above it. One possible chord sequence that fits this CR would be Bø7 followed by A7♭9.
The chord sequence written in functional harmony terms as ii-V7-I would be expressed in relativistic terms as a pair of CRs: m-IV-7 and 7-IV-M.
Applications
Musicians using this framework are encouraged to catalogue CRs according to what emotions they evoke in themselves personally, rather than how either chord would function within a given key. When writing a chord progression with a given emotional context in mind, any CRs that feel close to the desired emotion may be freely chosen, and the resulting sequence of chords, though not necessarily tonal in nature, will still tend to evoke the desired emotional context fairly closely. In a way, this approach treats each chord like a new key center and each chord change as a modulation.
Regardless of the musician's intention, the listener's ear will still want to lock onto a tonic. Harmonic relativity can disrupt this tendency, and while the resulting surprises can be musically interesting, care must be taken to ensure that all semblance of structure is not lost (unless you're going for that sort of thing, in which case have fun!) One common way of addressing this need is to create chord sequences that revisit the same chord, or chords within the same key, several times throughout a progression. For example, the progression written M-II-M, M-IV-m, m-IV-M ends on the same chord it starts on.
Voice leading is especially important to consider under harmonic relativity. Since emotionality is generated by each pair of chords, the effect of each voice's movement within a CR is heightened.
External links
This video series by YouTube user JJayBerthume is a good entry point to harmonic relativity.