Robert Plant on “Black Dog”: “My daughter’s boyfriend, who played in a psychobilly band, once told me there was something wrong with ‘Black Dog’—he pointed out a line in 5/4 time mixed in with the rest of the song, which is in 4/4. It drove me nuts. I grabbed the record, put it on, and said, ‘Listen, midget, this isn’t a mistake—this is what we were capable of!’”