Looks like Bert Weedon, who died last week at the age of 91, had a far-reaching impact on classic rock indeed. His 1957 book, ‘Play in a Day,’ influenced musicians like the Beatles, Eric Clapton, Pete Townshend and Brian May — and now you can add Black Sabbath members Geezer Butler and Tony Iommi to that list as well.

In paying tribute to the man many people called “Mr. Guitar,” Butler wrote on his website:

I had stopped commenting on deaths and disasters, because my [online] ‘journal’ was starting to resemble an obituary column. However, I cannot ignore the passing of Bert Weedon. Some of you won’t have heard of him, but I can safely say that many U.K. and Ireland guitarists from John Lennon onwards throughout the 1960s and ’70s would probably have never learned to play without Bert Weedon’s ‘Play In A Day’ book. He produced a book with pictures of chords, rather than musical notation, making it possible for those of us who couldn’t afford music lessons, or didn’t read music, to gradually learn basic chords on the guitar. I know BLACK SABBATH wouldn’t exist without his book, since both Tony Iommi and myself learned to play guitar chords from it. RIP Bert!

You can catch Butler, Iommi and the rest of Black Sabbath when they play their sole U.S. tour date at the Lollapalooza festival in Chicago this summer.