The leadoff track on Led Zeppelin's 1971 fourth album, "Black Dog," immediately kicks the band's undisputed masterpiece into high gear with a shower of fireworks from Jimmy Page and Robert Plant. Yet there's also a intricate complexity to the song that's easy to miss between all the power riffs and blues belting. The rhythmic accents between Page's guitar work and John Bonham's drumming never fall into a neat, straight-ahead pattern, playing around the beat instead of falling squarely on top of it every time. Page's long phrases ascend, descend, and pause at different intervals of time in different sections of the song; in fact, there are one or two spots where it sounds like he's nearly tripped over himself and has to rush a bit to catch up to the beat. Besides Page's busy riffing, the other crucial element of "Black Dog" is its stop-start structure. Plant leads off the song by belting out an unaccompanied, echoing blues couplet, and when he's finished, the band kicks in behind him, running through the main riff and then dying back down so that Plant can sing the next two lines. Page tacks some variations onto the main riff at a couple of points during the song, and there's also a different section built around power chords that marks the only part of the song in which Plant sings over full-band accompaniment. Page ends the song with a fiery solo over this riff while Plant moans and wails in the background. The sense of band interplay, the novel tradeoff structure, and the raucously heavy riffs all help make "Black Dog" a convincing demonstration of why Led Zeppelin was arguably the greatest hard rock band of all time.