> They're very rare in pre-romantic classical music...

Not true. Some styles of classical music employ dissonance in a prominent way - from late 14th century Ars Subtilior, through the pre-baroque stylus fantasticus and the bold chromaticism of Gesualdo, Frescobaldi, de Macke and others, and to the late baroque with Bach, Vivaldi, Zelenka, Rebel and others.

In traditional harmony something is considered "dissonant" if it's not part of the root triad in root inversion. All such chords are unstable, the theory goes, and must eventually resolve to some tonic chord. Classical harmony sticks almost exclusively to diatonic intervals, triads, and sevenths, and uses inversions and juxtaposition of chords (in sequence, via 'suspension', and so on) to provide "dissonance".

This isn't the kind of dissonant the OP is talking about. I'd be very surprised if you can come up with a clashing jumble like a Dom7#11 in Bach, even as a suspension or pedal. Cluster chords in classical music are almost unheard of.

"Traditional harmony" evolved after the Medieval and Renaissance music the parent post is referring to, which can sound rather radical by Classical Era standards, by which time the diatonic harmony you speak of dominated professionally composed music. I'm not am expert, but I understand Western folk music traditions remained pretty funky.

Not really. Western folk music is usually harmonically conservative. As for art music, the codification of classical harmony began with Renaissance theorists and the standard codifications are based on that lineage of thought. Counter-reformation composers like Palestrina were considered the most harmonically "perfect". The few true weirdos like Gesualdo were considered, well, really weird and thus excluded from common practice. Finally, most of the guys mentioned by the previous poster are actually Baroque.

I don't think even Gesualdo used chord clusters. IIRC it was his chromatic sense of tonality that was shocking, not his choice of chords.

You guys aren't disagreeing exactly. acjohnson55 mentioned "Medieval and Renaissance" but you're just talking about the Renaissance, and the Medieval-to-Renaissance transition is the interesting part (I think acjohnson55 just got the timing a bit wrong). The major third as a consonant interval became accepted in the early Renaissance. Most medieval composers considered thirds to be dissonant, which makes medieval music sound strange to modern listeners because things don't resolve as we expect them to, with phrases often ending on open fourths or fifths instead of triads (though to be fair, their third was somewhat different than ours because of changes in temperament over time).

* https://en.wikipedia.org/wiki/Wolf_interval

Indeed I find of their two examples the "consonant harmony" is a bit dull and the "dissonant harmony" sounds like it might be from something more interesting like the Große Fuge, one of my favourites and probably more dissonant than their example. https://www.youtube.com/watch?v=XEZXjW_s0Qs&feature=youtu.be...