One of Yes' most admirable qualities is their malleability – a crucial element of 1974's Relayer, released in the U.S. on Dec. 5, 1974.

The group has survived throughout the decades, despite the loss of several key members, largely because they've welcomed new blood with open arms. But fans and critics were rightfully skeptical following the departure of Rick Wakeman. The keyboard whiz – a lightning-fingered showboat prone to donning capes onstage – fled in May 1974, saying he'd become bored and creatively exasperated following the infamously excessive Tales From Topographic Oceans tour.

"I think we veered off the path with Tales because of various reasons," Wakeman told Hit Parader later that year. "And if I had stayed with the band, it would have veered off even more. It would have ruined the band and ruined a lot of good music. I think that because I have left, whoever will come in with them will now help the other four people pull it together, and they'll get back onto the path and continue to make really good music. [...] In the long run, it will undoubtedly be the best for them."

It seemed like wishful thinking. Wakeman's ornate, classically inclined playing was an essential element on early-'70s landmarks like Fragile and Close to the Edge. It was hard to imagine the band ever filling that massive void. And in the immediate aftermath of Wakeman's departure, they didn't bother. The remaining quartet of Jon Anderson, Chris Squire, Steve Howe and Alan White regrouped that summer at Squire's converted garage-studio in Virginia Water, Surrey, where they started writing and rehearsing adventurous new material.

At one point, the band auditioned Greek synth pioneer Vangelis, but a lack of chemistry – and, according to rumors, the keyboardist's fear of flying – quickly nixed the prospect. (Vangelis and Anderson would later team up for a string of electronic-based albums in the '80s.) This opened the door for Patrick Moraz, an eccentric Swiss player fresh from a stint in prog outfit Refugee, to join that August. Fleshed out as a five-piece, and working once more with longtime engineer Eddie Offord, Yes sought to venture into the sonic unknown – embracing the jazz-fusion bent of Moraz's synthesizers and a more free-form approach spearheaded by Anderson.

"I was very interested in doing something really modern," Anderson said in the liner notes for 2003 reissue of Relayer. "I wanted to do more electronic music – something radically different. I would talk to the band about doing free-form music, without thought. After Tales From Topographic Oceans, where the structure was so tight, why not do a piece of music so outrageously different?"

Listen to Yes Perform 'Sound Chaser'

And "outrageously different" is a perfect description for "Sound Chaser," a fusion-prog epic fueled by Moraz's jazzy keys and the manic rhythm section. White sounds particularly invigorated, playing with more explosive confidence than he did on Topographic, his recorded Yes debut. "People always ask me what my favorite Yes album is," White added in the Relayer liner notes. "From the perspective of where the rhythm section is coming from, I always single out the Relayer album."

Offord, in a 2002 talk with Notes From the Edge, said he welcomed White's arrival in Yes. "I always felt that maybe [original drummer] Bill [Bruford] had lacked a little bit of soul or something, but had great technique. But Alan, on the other hand, had a lot of feel and soul, but not enough technique. When he first joined the band, it was tough. It was really hard for him ... and then going into Tales with all of that uncertainty – the poor guy. It was really hard; it wasn't a solid situation. But having gone on tour and then coming back onto Relayer – Rick had been on the outside for a long time, anyway – so going into Relayer, Alan was more accepted and he was doing better, and this crazy Swiss guy was coming in. It was actually quite nice; it was a better time."

Relayer concludes with its most straightforward piece, the reflective ebb and flow of "To Be Over." The nine-minute track offers a melodic respite after the chaos of "Sound Chaser," fading into the "calming stream" with Howe's layered guitars, pedal-steel and sitar. But the album's clear centerpiece is opener "The Gates of Delirium," a 22-minute masterwork inspired partly by Tolstoy's War and Peace.

In its finished form, "The Gates of Delirium" morphs through multiple instrumental and vocal movements, including the hypnotic meditation "Soon" (which was released as an edited, stand-alone single) and a noisy mid-song "battle." "I just remember all kinds of weird percussion things Jon brought in, metal sheets and so on," Offord said of this bizarre section. "It was basically all created with percussion."

This large-scale epic began life as a series of abstract ideas in Anderson's head, which he attempted to translate to the rest of the band while playing rudimentary piano. "Jon actually led me through the compositions and through the core of the arrangement and the construction of most of the themes of 'The Gates of Delirium,' which were composed by the time I came in," Moraz said in the liner notes, comparing the track to a "symphony in the world of rock 'n' roll."

Listen to Yes Perform 'To Be Over'

After the Relayer tour concluded in August 1975, the quintet began work on new material. But this unique line-up wouldn't survive: Wakeman re-joined the band in 1976, pushing Moraz and his jazzy leanings to the curb.

"We had decided to do some writing – starting in 1975, when I was also helping Chris and Steve to record some music," Moraz told Something Else! in 2014. "We had started to compose and to co-compose and to gather material for what was going to be the album Going for the One, and I was very much involved in the composing of ‘Awaken’ at the time. I even recorded one or two tracks in the very, very beginning – in the early stages of sessions in 1976. I recorded some basic tracks for what was going to become ‘Awaken,’ and other tracks for Going for the One. Unfortunately, those were taken out, to allow Rick to come back to the band."

Showcasing a more concise and stripped-back direction, Going for the One was the band's final full-length masterpiece, offering fan-favorite singles like the aggressive title-track and the dreamy "Wondrous Stories."

While Going for the One itself remains an underrated prog-rock classic, that album at least had commercial appeal on its side. Relayer is the black sheep of Yes' '70s discography: too jazzy and abrasive for some traditional prog fans, too experimental and dense (aside from "Soon") for classic-rock radio. In many ways, it's a musical aberration – the first chapter of a strange, unfinished story. But that's one of the reasons it's still such a compelling listen.

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