I trust you’ve heard Krist Novoselic’s wonderfully sour bass notes under Kurt Cobain’s exclamations “Hey!” “Wait!” in the chorus of “Heart Shaped Box” (starting at 0:49).
The sourness is the tone of the whole song and of Nirvana the band, not as stretched out in a darkly exuberant chord progression or random segues in the lyrics but darkly exuberant and random all at once, brought to a point, a supremely concentrated hook.
The two questions on this subject that nag at me are, What exactly are these two notes? (They’re so odd and misdirective that they’re hard to place on the musical scale.) And which of the two is the greatest bass note in rock? (Under the rules of this Hooks post it has to be one of them. Right now I can’t think of any other bass note that has so much impact in its moment. Can you?)
The song is played in D (tuned low to D-flat, a nice depressive touch, but let’s treat it as D). The chords of each chorus line, same as the chords of the verse, are V-IIIb-I7, that is, A-F-D7. On the A chord, that bending-down bass note after the A is F-sharp to F; on the F chord, the counterpart is a bending-down C-sharp to C.
F-sharp and F are about as lost in space as any notes could be if your home ground is an A chord. In a bass part, the strongly relevant notes you should hear for A are A the first, E the fifth, C or C-sharp as the third—those are the notes that make an A chord—or, getting a little more slick, you might hear G or G-sharp as a passing tone to get to A, or D or E-flat to get to E, or B to get to a C. Thus F-sharp and F live in the largest out-of-bounds area in the normal A scale, and by making a pair of them, not sliding off of F-sharp to the more rock-wholesome G but sliding down to F as the very worst note possible (apart from B-flat, but let’s not even think about that), the bent bass note is confirmed in maximum weirdness.
Why doesn’t this ungodly F-sharp-to-F sound like a clam? Partly because it’s down in the bass line and you don’t hear the notes, even in this way-out-front bass voice, quite as distinctly as you would in a vocal or lead instrument part. Partly because the A-F-D7 pattern has already been strongly established by the verse, so you know you’re headed for an F chord next, so the F note might be an advance scout for the F chord, just as the F-sharp might represent the D7 chord that comes at the end. (I know . . . after all, no hook is an island.) Partly because starting a bent note on F-sharp would work out okay if it went up to G, which is the seventh note of an A7 chord, and going down to F instead sounds (in this song) like an aptly demonic inversion of that move.
Arriving with the F chord, the second sour note is similar but different. On its own F scale it’s a half-step lower: it goes from the sharped fifth to the fifth rather than from the sixth to the sharped fifth. So there is a subtle feeling of contraction, of being reeled in closer to the normal dimensions of a chord, the better to appreciate the D7 chord at the end of the line. Also there is a bit of anchoring, since we are ending the halftone bend on a solid fifth this time. Most importantly, perhaps, by ending on C we’ve previewed the F-sharp to C sequence that will be the climactic melodic hook of the whole line on the D7 chord (the notes of “I got a new/complaint”).
Can we decide now which note is greatest? The first note is the craziest. It announces something, but you don’t know what. It brilliantly throws you off. (Actually it’s a bigger gesture with a wilder curve than the F-sharp-to-F analysis recognizes.) The later note is craftier, still sour but a little more solid in its harmonic space. It’s the earlier note’s slightly less dangerous brother, the one you’re better off trying to reason with. I’ll risk my sanity and go with the earlier note, because it sets everything up in that line, which is that song, and that band—a colossal Wrong Is Right.
 Close: the low G at 1:26 in Jefferson Airplane’s “Other Side Of This Life” on Bless Its Pointed Little Head, like a bottomless dark pool you finally discover in the middle of a cave full of fantastic bass note stalactites and stalagmites.