A perfect 5th is not the same as a diminished 6th unless we assume equal temperament tuning. Granted it is the dominant tuning, but it irks me when this is just silently assumed.
Plenty of music around that is recorded using actual perfect intervals, so why muddy the waters?
I firmly disagree. I learned about staying "in tune" with those that I was playing with in an ensemble long before I learned about equal-temperament and its concessions to multi-key harmony.
I'd say removing the beats from your partials is way more fundamental to both music making and music theory than chromaticism. Chromaticism is the next step, beyond basic music theory.
> I learned about staying "in tune" with those that I was playing with in an ensemble long before I learned about equal-temperament and its concessions to multi-key harmony.
I'd consider that a performance technique before a theory aspect, like vibrato speed and control or enharmonic fingerings.
Consider this: If you were in a duet as a beginner and the sheet music had your partner playing a C and you playing an A double-flat, how would you be instructed to play it?
You'd be told it was enharmonic to a G, and play it as a G.
Until you start reaching deep into historical re-enactment or advanced theory, it's very safe to assume equal temperament and leave the ear-adjustment to performance.
I guess my response to this is that basic music theory is roughly equivalent with basic acoustics, has more bearing on generalized musical practice than what you are implying, and that even reading sheet music is an abstraction that requires foundations in a musical culture that has a prerequisite of certain assumptions that may not actually hold.
If you listen to CPE Bach knowing that each note can be bent (as on a guitar, because it is a clavichord), then the written music makes more sense because each note can be tuned to be harmonic with the fundamental. The sheet is just a sketch. The presumed required bend in each note totally changes the expectations of the key it is written in.
Or, if you are listening to a gamelan, then the beating of notes becomes an essential rhythm of the instrument, informing the tempo of the ensemble as a whole.
Music theory is a combination of acoustics and music history, but the acoustic part is more fundamental/basic. Like knowing "Clueless" is based on "The Taming of the Shrew" is informative, but the fundamentals of quality movie making or movie consuming do not require you to know anything about Shakespeare.
A fifth might even sound off key if you're very used to equal temperament (it's about 2 cents below an equal temperament). You know it by there being no or less "wobbling" between the tones.
For listening tips, look for vocalist groups where there's "One Voice Per Part" (OVPP). Voces8, Vox Luminis, etc. When there's only one voice, you don't get the inherent wobbling happening when two instruments/voices play in unison.
Not all genres are possible to have just (jazz chord colors would sound rubbish).
You basically can look up just intonation versus equal temperament for the basics. https://pages.mtu.edu/~suits/scales.html gives the mathematical answer but doesn't get into the history.
A clause that says "assuming twelve-tone equal temperament" would be sufficient here, but you can really go down the rabbit hole if you start digging into scales (see microtonal), and your page is meant to be more basic.
Plenty of music around that is recorded using actual perfect intervals, so why muddy the waters?