Short answer — tritones and distortion

Longer answer — many opinions within the heavy metal community

The Unholy Trinity of Protometal — Led Zeppelin, Black Sabbath, Deep Purple (though nowadays people consider Led Zeppelin to be “classic rock”)

Heavy Metal music was written during the 60s to late 70s — “Much of the development after that, while being both heavy and metal, isn’t considered Heavy Metal” by musicians in the genre. Taxonomy!

Riff based featuring tritones, half steps, flat 9s, flat 6s, “and other dissonant and atonal gestures”

“Good riffs can sound heavy even if you play them acoustic” — not just a matter of tuning everything down. But, “though dissonance is the seed of heaviness, it’s not the whole story”

This is an especially good episode, I hope to come back to it and take more notes!