Brilliantly episodic, and strangely (for Led Zeppelin, at least) psychedelic, 'What Is and What Should Never Be' ultimately delineates the two faces of this group like few other individual tracks: Folky and ruminative one moment, this standout cut from 1969's 'Led Zeppelin II' is just as brawny and tough the next.

Of course, all of it is couched in this of-the-moment trippy weirdness. Clearly in awe of the period's newly discovered recording technology, 'What Is and What Should Never Be' has a kitchen-sink attitude about things: Robert Plant dabbles with phased-out vocals, while Jimmy Page's guitar sweeps from side to side across the entire stereo spectrum.

Still, that takes nothing away from the darkly intriguing, rather incestuous theme that runs through the song, beginning with a brief, whispering flirtation before Plant simply erupts -- along with Led Zeppelin -- for a moment of dizzying hedonism. Then, just like that, Page leads them back to a contemplative vibe. The wonder of 'What Is and What Should Never Be,' in that moment, is the delicacy and emotional reserve that John Bonham uses. Whatever his justifiable reputation as a stick-breaking pounder, cuts like this one go a long way toward illustrating the depths of his talents at the drums.

Keep your seat belt fastened, however. Before long, 'What Is and What Should Never Be' has topped the next vista and Led Zeppelin is off on a rumbling ride again. And so it goes until finally, with about one minute left, they throw out the playbook for a edgy, horizon-to-horizon riff by Page -- providing a platform for this cathartic groove that closes things out.

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