Everything changed when Tony Iommi cut the tips of his fingers in a factory accident and had tune his guitar lower resulting in the signature Black Sabbath sound
Paraphrased from an interview I read in a guitar magazine years ago with Tommy Iommi:
"It was the strangest thing, I would lower the tuning and get a deeper sound, and then Ozzy would sing higher than he did before!"
I didn't know that. Went this year to see Ozzy + Black Sabbath in Los Angeles, then Iron Maiden later, but the biggest event I was was David Gilmore (from Pink Floyd) - the sound (at the Hollywood Bowl) was so amazing, that when I recorded it, and sent it to my father, he couldn't believe it was live. And my father, since he bought me Heaven & Hell when I was teenager, and had lots of time listening to Wish you were here, Welcome to the machine and many other crazy songs in the car.