CONSONANCE / DISSONANCE
For a really fascinating general discussion of changing perceptions and historical presentations of what consonance and dissonance mean, check out this. Especially the last few pages. Also there is a hella cool appendix.
Read especially the section towards the end that says
"Section VI Summary and Conclusions: Toward a New Terminology."
Here is a summary of the summary:
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In the course of the two-and-a-half millennia since Pythagoras, the entitive referents for 'consonance' and 'dissonance' have changed from melodic intervals (in CDC-I), to simultaneous dyads (in CDC-2 and CDC-3-eventually extended to larger aggregates as well), and then to individual tones in a chord (in CDC-4), and finally to virtually any sound (in CDC-5). . . .
In the earliest form of the CDC-which I have called CDC-1-the terms 'consonance' and 'dissonance' had an essentially melodic connotation, referring to a sense of affinity or relatedness between the pitches forming an interval. The consonances were those intervals which were directly tunable: the perfect fourth, fifth, octave, and the octave-compounds of these. All other intervals were considered dissonant.....
With the advent of polyphony in about the 9th century, a new conception of consonance and dissonance emerged-CDC-2-which had to do with an aspect of the sonorous character of simultaneous dyads. In its earliest rnanifestations, this new form of the CDC was only barely distinguishable form its predecessor, because in the earliest forms of polyphony only the consonances of CDC-1 were used to form simultaneous aggregates. With the increasing melodic independence of the added voice or voices in the loth, 1 lth, and 12th centuries, however, the category of consonances was gradually expanded to include thirds and (by the same process of expansion, though not until sometime later) sixths. ...
New developments in polyphonic practice in the later 13th and early 14th centuries-including what came to be called "the art of counterpoint"- eventually led to a new system of interval-classification, and a new conception of consonance and dissonance which I have called CDC-3. This form of the CDC seems to have been shaped by two factors: (1) a tendency to reduce the number of distinctly labelled categories to a smaller set which would have an operational correspondence to the rules of counterpoint, and (2) the emergence of a new criterion for the evaluation of consonance and dissonance. As a result of the first of these factors, the five or six perceptually distinct categories in CDC-2 were reduced to three operationally distinct categories: "perfect consonances" (octave and fifth), "imperfect consonances" (thirds and sixths), and "dissonances" (all others, including the perfect fourth)....
CDC-4: In this form of the CDC, any note which is related to the harmonic root of an aggregate as prime, third, or fifth-i.e. any note which is a triadic component-is a consonance (or consonant note), while any note which is not thus related to the harmonic root is a dissonance (or dissonant note). . . .
CDC-5. In this form of the CDC-first clearly articulated by Helmholtz in 1862-the dissonance of a dyad or larger simultaneous aggregate is defined as equivalent to its "roughness," and this turns out to be dependent on pitch register, timbre, and intensity, as well as on its constituent intervals.