What is jazz-rock? The label has been applied to the work of diverse artists such as Chicago, Blood Sweat & Tears, Miles Davis, Steely Dan, and Santana (and that's just to name some of the most well-known exponents) not to mention an entire cadre of fusion artists like the groups Weather Report and Return to Forever. In the jazz-rock canon, the name of Frank Zappa certainly stands tall. The multi-faceted artist delivered one of the genre's earliest and most seminal albums with 1969's Hot Rats. Like so many of Zappa's works, the LP defies simple description. Working closely with multi-instrumentalist Ian Underwood and an A-list of veteran musicians, Zappa took full advantage of 16-track recording, then in its infancy, to meld and juxtapose jazz's compositional and improvisational sophistication with his original brand of rock (itself inspired by rhythm and blues) and signature full-throttle electric guitar.

Late in 2019, Zappa Records and UMe marked 50 years of Hot Rats with a dazzling 5-CD box set. The Hot Rats Sessions is a captivating look behind the curtain at this singular, primarily instrumental album. It mainly documents just four productive days - July 18, 28, 29, and 30, 1969 at T.T.G. Studios in Hollywood (overdubs were recorded later) - recording both compositions and improvised jams. The first three discs chronicle the album's basic tracking sessions while the final two discs add up to a Hot Rats potpourri with Zappa's complete 1987 album remix, single masters, radio ads, and alternate mixes in various stages of development. (All that's missing is the original 1969 album, though a number of its raw master takes are included. It remains available as a standalone disc from Zappa/UMe.)

Recording in multiple studios, Zappa enlisted different groups of musicians to achieve its varied sounds and textures. Hot Rats struck a democratic balance between rock and jazz, synthesizing the best qualities of both into a spellbinding whole. Its six songs encompassed electrified chamber music ("Little Umbrellas," "It Must Be a Camel"), quirky, majestic soundtracks in miniature ("Peaches en Regalia," "Son of Mr. Green Genes"), dirty blues ("Willie the Pimp"), and jazz-funk ("The Gumbo Variations").

Immersing oneself into the six discs of material here, Ian Underwood's contributions truly come to the fore - not least of all on the two warm and stately solo piano outtakes that open this set. Underwood played 13 separate instruments on the album (plus finger snaps!) and worked closely with Zappa to shape and bring these musical visions to life. There's symmetry to the sequencing as two sections of "Piano Music" from July 18 are the first two tracks on Disc One, and the overdubbed version of Section 3 concludes the main portion of the box on Disc Four. The latter version is positively shimmering: playful, baroque, and haunting. As dedicated fans know, Zappa never threw anything away; his "conceptual continuity" led to the two piano pieces cropping up on Burnt Weeny Sandwich the next year.

Producers Ahmet Zappa and "Vaultmeister" Joe Travers have sequenced the first four discs in chronological order based on the three days of recordings. Not everything recorded for Hot Rats is here, but this curated selection (accurately noted as "essentially everything" by Travers) does represent every track. The original 1969 LP mix isn't included from start to finish, but when a master take (whether of a section or a complete song) appears, it's helpfully indicated in the discographical annotations.

The six tracks weren't written and recorded in the conventional manner. Instead, a laser-focused Zappa crafted each song in a close collaboration with his hand-selected musicians. One can sense and share in his joyful sense of discovery on July 28 as the elements coalesce on the mesmerizing "Peaches en Regalia," one of the composer-guitarist-bandleader's most accessible and melodic works. It manages to be both compositionally complex and seemingly effortless, and here are its building blocks with just the killer rhythm section of Underwood, teenaged bassist Shuggie Otis, and drummer Ron Selico. (Shuggie's father Johnny Otis and Don "Sugarcane" Harris join in for an extended, alternately bluesy and funky jam on violin and tack piano, respectively.) Zappa's interjections are instructive: "More fills, get loose!" or "Come on, get vicious!" Some overdub sessions, such as for Underwood's horn parts, are absent, but the finished song can be heard both in its 1969 mono single mix and 1987 CD remix.

The July 29 session welcomed violinist Don "Sugarcane" Harris, Ian Underwood, bassist Max Bennett, and drummer John Guerin. (The latter two would later support Tom Scott in the L.A. Express, working with Joni Mitchell and others.) Bennett, a mainstay of the West Coast jazz school, found his style remarkably adaptable to rock. Guerin had worked with Zappa as far back as 1963 and played on 1968's Lumpy Gravy. Zappa would make full use of his distinctive fills on the tom-toms and expressive style. The moody, languid "It Must Be a Camel" is aired in its raw rhythm section track before its jolting percussion overdubs were added. In one fly-on-the-wall moment, Zappa requests that Guerin "destroy the mood completely" and play like a "raging monstrosity." The pensive "Natasha" (heard once in a session rehearsal and then in its master take) became "Little Umbrellas" with substantial overdubs adding its almost Indian flavor. Leaving no stone unturned, the "Cucamonga Version" recorded circa 1961-1964 offers an embryonic rendition of the tune.

The lone vocal track on Hot Rats (featuring guest blues shouter Captain Beefheart), "Willie the Pimp," is one of the original album's three lengthy jams. While "Willie" ran around nine-and-a-half minutes on the LP, but The Hot Rats Sessions presents that scorching instrumental master take - sans Beefheart's vocal contributions - in its unedited form of over 15 minutes in length. Two guitar overdub takes, one approaching the 15-minute mark itself, are positively hypnotic as they put Zappa's virtuosic axe squarely in the spotlight. Harris also shines with his fast and furious rock violin.

"Son of Mr. Green Genes" is one of Hot Rats' enduring centerpieces with its sly musical humor, orchestral pomp, and rock urgency. Take 1 showcases Zappa's fluid, funky guitar and its interplay with the rhythm section of Underwood, Bennett, and drummer Paul Humphrey; even in this early stage, it's clear that the composition is a cousin of "Peaches en Regalia." The unabashedly funky "Gumbo Variations" was cut on the same date as "Son of Mr. Green Genes" on July 30.  It's here under the title of "Big Legs" in a version almost twice as long (over half an hour) as the album cut, giving aficionados the chance to savor even more of Zappa, Underwood (blowing a storm on his tenor sax), Harris, Bennett, and Humphrey as they trade off, solo, play off one another, and otherwise epitomize a rock/funk/jazz jam mélange.

Those original six pieces are just the tip of the iceberg of The Hot Rats Sessions. Numerous outtakes premiere here, most of which will be familiar to Zappa's faithful. He, Shuggie Otis, and Ron Selico hone the tempo and approach, from buoyancy to urgency, on the fiendishly catchy and complex "Arabesque." It was subsequently overdubbed and reinvented as "Toads of the Short Forest," and Zappa's guitar overdubs are included on a separate track.

Two versions of "Dame Margret's Son to Be a Bride" are led by Underwood's groovy organ tightly linked with Otis and Selico: the first is driving and almost danceable while the second is just a bass-and-drums track. (FZ wiped its other parts when he incorporated it into "Lemme Take You to the Beach" on Studio Tan.) "Bognor Regis," a jam with tasty blues licks from FZ and violin from Sugarcane Harris, was developed into "Conehead" as heard on 1981's You Are What You Is.

Numerous takes of "Transition," exploring the right tension between Ian Underwood's late-night piano and John Guerin's aggressive drumming, culminate in a complete version. Zappa's direction towards Underwood, Guerin, and Max Bennett during these takes is among the set's most fascinating chatter, and "Transition" itself is a central theme which lodges itself in one's head and stubbornly refuses to leave. With overdubs, a portion would become "Twenty Small Cigars" on Chunga's Revenge.

Frank Zappa edited Hot Rats with surgical precision, and it's clear to see why some of the tracks ultimately didn't make the cut on the album. The rollicking "Lil' Clanton Shuffle," remixed and edited by roughly two-thirds for a 1998 debut on The Lost Episodes, is presented in its full form. The bluesy shuffle led by Harris' violin makes for one of the most straightforward items attempted, and it's an odd match for the original LP's other tracks. So is the down-and-dirty Little Richard cover, "Directly from My Heart to You." Though it was edited for its appearance on Weasels Ripped My Flesh, the full, unedited master take is here including Harris' lead vocal. Don Preston, Roy Estrada, and Jimmy Carl Black of The Mothers of Invention played on this track even as the group was breaking up. The same group plays on the epic, for-diehards-only nearly half-hour-long "Another Waltz," portions of which ended up in "Little House I Used to Live In" on Burnt Weeny Sandwich. The Mothers were on fire; earlier in the day, Zappa had reportedly prepared the press release announcing the group's dissolution and let his bandmates know.

The "From the Vaults" section on Discs 5 and 6 is a potpourri of relevant material: radio spots, single versions, alternate mixes from 1969-1972, and a couple of tracks that could have been sequenced comfortably in the main body of the box (Captain Beefheart's a cappella, gutbucket vocals for "Willie the Pimp," guitar tracks for "Arabesque").

Craig Parker Adams has newly mixed all of the music here from the original multitracks, and Bob Ludwig has mastered. Everything is handsomely packaged in a glossy, LP-sized box with a lift-off lid. The discs are slotted in a sturdy folder with images of the original tape boxes and tracking sheets. A "Zappa Land" board game with playing pieces and instructions is also included ("Frank needs to get to the studio. The first player to get him there wins the game!") as well as bag of guitar picks. While FZ would likely have approved of this eccentric touch, one needn't play the game to go to Zappa Land...the box set itself is the best journey there. A 28-page softcover booklet contains copious photos of the sessions, track-by-track annotations, an appreciation by The Simpsons and Futurama creator Matt Groening, notes from Ian Underwood and cover photographer Andee Nathanson, and a closing essay by Joe Travers. Those wishing to delve even further into the world of the album should seek out the newly-released The Hot Rats Book by Ahmet Zappa and photographer Bill Gubbins, featuring even more rare session photos and interviews.

Frank Zappa's discography can be daunting, but a dip into the original Hot Rats (still available on standalone CD from Zappa/UMe) can be a solid place to start. Chances are, you'll then want to head here for more. The composer described the six songs on Hot Rats as "amazing rock 'n' roll concertos" and, collectively, "a movie for your ears." But whether you're into rock, jazz, or just plain interesting instrumental music, Hot Rats retains the power to fascinate. This box set vividly raises the curtain on its creation, and in doing so adds another valuable chapter to the Zappa recorded legacy.

The Hot Rats Sessions is available now at:

6CD: Amazon U.S. / Amazon U.K. / Amazon Canada
LP (Original Album Only): Amazon U.S. / Amazon U.K. / Amazon Canada