The Beach Boys' sunny vocal harmonies are one of the signatures sounds of the modern era. Among rock and roll groups of the Sixties, the California quintet place second only to the Beatles in terms of their overall impact on the Top Forty. They were the Fab Four's only serious competition on a creative level, too. Paul McCartney has allowed that the Beatles' masterpiece, Sgt. Pepper's Lonely Hearts Club Band, was their attempt to rise to the challenge of the Beach Boys' magnum opus, Pet Sounds (which itself was inspired by the Beatles' first self-contained album, Rubber Soul). This creative dialogue between the two biggest bands of the Sixties pushed rock and roll to its artistic apex. Recently, Paul McCartney noted that "both [Pet Sounds and Sgt. Pepper] have more than held up. To me it's like, 'What have people been doing in the meantime? Where's the progress?' I can't see anything as modern as that around at the moment."