One of Metallica's most famous riffs might never have existed if not for Soundgarden.

"Soundgarden had just put out Louder Than Love. I was trying to capture their at­titude toward big, heavy riffs," guitarist Kirk Hammett told Rolling Stone in 2008. "It was two o'clock in the morning. I put it on tape and didn't think about it. When [drummer] Lars [Ulrich] heard the riff, he said, 'That's really great. But repeat the first part four times.' It was that suggestion that made it even more hooky."

And so begins "Enter Sandman," which became a million-selling single then a fixture at sports events. A connection between the two bands continued. They made a memorable – and somewhat controversial – appearance together at Lollapalooza, and both were set to play last weekend's Rock on the Range. Then came news of the shocking suicide of Soundgarden singer Chris Cornell.

Now they're joined in grief. Metallica addressed the sad news during their May 19 show in Foxborough, Mass. "We forgive you, Chris," frontman James Hetfield said after the band played "The Unforgiven," another standout cut from Metallica's 1991 self-titled album. Later in the same show, bassist Rob Trujillo referenced Soundgarden's "Black Hole Sun."

Some fans questioned Metallica's headlining spot at 1996's Lollapalooza, but the heated debate ended up giving Hammett yet another opportunity to note his Soundgarden fandom.

"I think that the bill on this year's Lollapalooza is pretty good," Hammett told Guitar World back then. "It may be a little top-heavy because of us and Soundgarden, but it's certainly stronger than last year's. F--- all those f---ing elitists who say 'Metallica's not alternative' or 'They're too big of a band to play Lollapalooza.' They're just being very narrow-minded. I've been to every single one; I've actually jammed at a few too. When Ministry was out, I played with them a few times, and I did the same with Primus. I've f---ing loved Soundgarden since 1985 or '86."

Metallica kicked off their WorldWired tour behind Hardwired ... to Self-Destruct earlier this month. Cornell died on May 18 just hours after Soundgarden played a Detroit concert. Survivors say prescription drugs may have played a role in his death.