The sound of "JUST INTONATION"

Nearly all modern music is composed and performed in 12-TET, and music theory is almost presented that way as well. Just intonation, when mentioned at all, is often treated as a historical footnote. This isn't to say that all modern music rigidly conforms to equal temperament; instruments with contiuous pitch (voice, violin, trombone, fretless bass, etc..) can accomodate JI just fine, and performers may gravitate towards just tunings instinctively without even being aware that they're doing it.