The first sound you hear on Led Zeppelin's fourth album, which begins the No. 4 song on our list of the Top 50 Led Zeppelin Songs, comes from Jimmy Page's guitar prepping for action. Distorted and multitracked, Page's Les Paul takes tentative steps at first, like it's not quite sure what's coming next. It's a beguiling start to Led Zeppelin's best, most popular and, at the time, most assured album.

From the instant Robert Plant steps in with an a cappella promise -- "Hey hey, mama, said the way you move, gonna make you sweat, gonna make you groove" -- 'Black Dog' kicks like very few songs in Zep's catalog. Its stop-start structure and arrhythmic time signature are way more complex than they let on. After the stripped-down 'Led Zeppelin III,' the untitled fourth album brims with plugged-in inspiration.

Of course, the song has nothing to do with a dark-colored canine; like most Led Zeppelin songs, it's about sex (according to legend, a black Labrador was hanging around the studio during the sessions). Everything about 'Black Dog' -- the riff, the moans -- drips with hot, nasty, dirty lust.