It's remarkable when you think about how close the "Big Four" thrash metal bands are in origin.

After all, Dave Mustaine originally played in Metallica before he formed MegadethKerry King was in an early iteration of Megadeth before he fully committed to Slayer. And both Metallica and Slayer made their recorded debuts courtesy of Metal Blade Records. It's all California metal history of the highest tier.

Diehard thrash fans know well the early tales of all four acts (the fourth being NYC's Anthrax), especially Metallica's emergence with "Hit the Lights" on Metal Blade's 1982 Metal Massacre compilation. It was followed by Slayer's "Aggressive Perfector" on 1983's Metal Massacre III. How much closer could you get?

In proximity, anyway. Brian Slagel claims Metallica and Slayer weren't close otherwise. Except for when it came to trying to spy on each other through him.

Slagel is the founder and CEO of Metal Blade who helped the thrash icons all those years ago, and continues to keep an eye on metal's future.

"Metallica and Slayer were never very close," Slagel tells Metal Hammer. "They were competitive. I was friends with both bands, and Metallica would ask me, 'What are Slayer doing? What are they writing?' And the Slayer guys would ask me, 'What are Metallica doing?'"

He continues, "It was, 'Who's faster? Who's heavier?' They actually played together early on in Orange County [Calif.]. It was really early in Slayer's career — it was probably only the second or third gig they had played."

King recalls his first time seeing Metallica at Woodstock Concert Theatre, also in Southern California. With Mustaine still on guitar before being replaced by Kirk Hammett. But rather than remembering any rivalry, he cites Metallica as an influence on Slayer.

King says, "I like speed metal or thrash metal — which hadn't been named yet — which is what they were to me. I liked what they were doing with it, and I was already into Venom. I think that a cross between Venom, [Judas] Priest and Metallica kind of made Slayer what Slayer is."

He adds, acknowledging the established musical foe of thrash, "Rather than them being competition at that point, we were all competing against glam."

Glam or not, thrash metal has further blossomed and branched out since 1983. And fans have Metallica, Slayer, Megadeth and Anthrax — and plenty of impactful thrash from bands not in the Big Four — to thank for it.

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