The seven string guitar has existed in many iterations over the years, though finally had its first mainstream electric production model as the Ibanez UV7 in 1990. The Ibanez UV7 was made for Steve Vai prior to 1990 and was used on both Whitesnake's Slip of the Tongue and Vai's own Passion & Warfare. Vai tells Ultimate Guitar the model quickly faded into novelty and obscurity, though he encouraged Ibanez to keep making them.
Then Vai heard Korn using them and his mind was totally blown.
"What I didn't realize was that there were a lot of young players at that time, who were unknown, listening to [what I was doing]. Some were attracted to what I was doing with it, and some were attracted to the idea of a seven-string guitar and may not have been very interested in the way I played.
"So I knew that somewhere, these young kids were going to get the concept of the seven-string and do things with it that were beyond what I was doing. Then what happened was the sale of the seven strings dropped to almost nothing because the wave of me using it had kind of come, and it was more of a novelty. But during that wave, there were these young guys buying it and writing music.
"Even when they weren't selling, I told Ibanez to just keep it available, even if you only sell a few a year.
"Then, I'm driving down the street and this song comes on the radio and I'm like, 'What the heck is that?', and I pull the car over. It sounded so heavy, I instinctually knew it was a seven-string and somebody was doing something with it that was much different than what I was doing.
"And that band was Korn. That was sort of the rebirth of the seven-string. So it was a co-creative effort. I was a part of it but it took many, many people to bring it to the level where it's at.
"So it's really nice to know you have made a contribution that others have taken and run with it. That's what I do, I just take ideas and mix them up in my head to how they could serve me best and then manifest them.
"The idea that I'm associated with something like the seven-string, or the RG, and even the JEM, for a lot of people, they don't know it, and that's fine.
"I do interviews sometimes and I've been asked several times, 'With everybody using seven-string guitars, do you think you'll ever use one?' and my stock answer is, 'Nah, it's been done already.'
"So it's nice to see, it really is. It's nice to see that in other areas as well. There are certain technological parameters in other gear that I know I contributed to. I'm not patting myself on the back… maybe I am, I don't know. Do you remember the 969 Harmonizer?"
So there you go. Steve Vai accidentally invented nü-metal. You're welcome.