This Friday is the 30th anniversary of Nirvana’s groundbreaking Nevermind, an album that forever transformed the pop music landscape. The album’s furious rock anthems “Smells Like Teen Spirit,” “Come As You Are,” and “Lithium” made everything else being played on the radio at the time seem scrawny and frivolous by comparison. In the ensuing three decades, countless words have been written about how Nevermind mainstreamed the sound of grunge by emphasizing emotional delivery over tonal precision. And music journalists have spilled gallons of ink — real and digital — describing how Nirvana’s loud-soft-loud formula drew from punk and alternative rock (e.g. Pixies), hard rock (e.g. Boston), and early metal (e.g. Black Sabbath), while inaugurating new levels of dynamic contrast. So there’s no need to examine any of that here. Instead, let’s focus on something rarely discussed, but extremely vital: Kurt Cobain’s harmonic language.