In the late ’70s, punk rock was regularly credited, by its practitioners and champions, as the big reset button rock music needed — a severing with the past and musical orthodoxy that would embolden a new wave of innovators to find its own ideas. But the catch turned out to be an identity still open to compromise: after all, punk rock is still rock, with the stars and the rules and the tropes that come with it. And a generation later, as the promise of punk found both commercial codification in alt-rock and a retreat to ambivalence in indie, a new movement sprang up that did what it could to course-correct the whole idea of transcending the old rock notions.

No iconic faces, no familiar three-chord tropes, no guitar-bass-drums-only conservatism. This music still acknowledged that rock had happened, but note the past tense — this was post-rock. The inverted offspring of prog, swapping out showy Euro-classical flourishes for a more jazz- and krautrock-borne reliance on rhythm and groove, post-rock was clarified as its own idea by critic Simon Reynolds in the mid 1990s. “The bands — Pram, Techno Animal, Laika, to name just a few — grew up with rock, but now they operate in a post-rock hinterland, where anthemic choruses and adrenalized riffs are rarer than hens’ teeth.”

That’s how Reynolds introduced Tortoise to the readers of Spin in November ’95, and two and a half years later the promise of that band as instrumental post-rock standard-bearers took a surprising twist with their album TNT. Released on Thrill Jockey 20 years ago this Saturday, Tortoise’s third studio album showed that this band, already restless with the constraints of the traditional rock methodology, were proving to be restless with themselves, too.

As a project that started out with the hopes of becoming a rhythm section for hire, the core that bassist Doug McCombs and drummer John Herndon built around themselves always felt more beholden to feel than genre. Tortoise’s ’96 breakthrough masterpiece Millions Now Living Will Never Die unfurled so elaborately — especially in its near-21-minute opener “Djed” — that it smothered the ambitions of wonky style-mappers with a pure sense of mood manipulation. A cult of something more elusive than personality, Tortoise’s members remained laudable for their musicianship without the expectations of overshadowing it; as a band of ideas they’d rather invite interpretation than answer demanded explanations.

And that’s how TNT became one of a handful of classics the indie world recognizes them for. As an ensemble work, it benefited greatly from the actual changes in the ensemble itself: guitarist Jeff Parker, a jazz-rooted guitarist and alumni of Chicago’s legendary avant-garde organization the Association For The Advancement Of Creative Musicians, joined the group right as they were drastically expanding the notions of what they could express. Parker gets a good amount of credit for highlighting and reinforcing Tortoise’s jazz leanings, and earns it; his clean, chiming guitar notes are some of TNT’s most characteristic statements.

But it’s also the sound of a band reacting to hearing their music pulled apart and rebuilt, as the lingering effects of experiencing their Millions work’s transformation on 1996’s Remixed inspired drummer/engineer/producer John McEntire to take a continuously reconstructive approach to putting the album together. The band never played anything as a full ensemble — one member would come in, riff for a bit, loop it, and then let another member come in to riff off the riff, like improvisation in stop-motion. But for a record that the band had to completely relearn to play as an actual group effort during their subsequent live tour, TNT was built and rebuilt and broken down and reshaped so continuously over the course of the yearlong recording sessions that even the glitchiest moments have all their mechanical components covered in the busy fingerprints of working musicians.

The final release, then, holds this strange space somewhere between the digital and the analog that never really bothers to differentiate the two, and reverberates so deeply through its textures — guitar, marimbas, basslines, strings, brass, woodwinds — that the “how” of the music is totally eclipsed by the “where.” In their increasingly eclectic idea of what constituted a musical identity, their easily identifiable genre traits (dub, krautrock, jazz, drum’n’bass, et al) felt less important than where they would take you. It’s almost cliche to call it a travelogue — the titles hint as much (“The Equator”; “Jetty”; “Everglade”; “The Suspension Bridge At Iguazú Falls”), but the sounds do even more, evoking the cultural landscape of international sounds at the same time as they erase the borders.

“I Set My Face To The Hillside” triangulates the folk sounds of Latin guitar, the Ennio Morricone scores that drew on them for tension-building inspiration, and the Augustus Pablo productions that riffed off those to turn cinematic wide-open desert vistas into lush musical greenery. “Ten-Day Interval” and “Four-Day Interval” echo like Steve Reich given more elaborate versions of the time-warping treatment NEU! put “Super” through, a dub of itself that creates an internal comment on how those marimba loops, piano chords, and basslines melted into each other. And the song they give the most impenetrable title — “In Sarah, Mencken, Christ, And Beethoven There Were Women And Men,” a nod to Robert Ashley’s experimental spoken-word tour de force — is also its most danceable one, a feint towards drum-machine indie-pop that plays out into something more breathtaking, King Sunny Ade’s Nigerian ju ju and the early microhouse itinerary of Kompakt mingling in the haze.

And those combinations, while recognizable, don’t take long to sink in as their own notions of new music, where decades’ worth of accumulated musical experience find both new evolutions and surprising compatibilities not for curations’ sake, but for emotion’s. Play TNT to put your mind inside a muso’s space of technique and craft, and you’ll eventually be redirected and lulled into a place where the nuances of what that music makes you feel start to push to the forefront. The opening title track feels daring enough at first when it comes to pitting surprising performance styles against each other — oh, I see, that delicate guitar figure is playing over high-energy drum fills — but like the instrumental hip-hop and dance music the album subtly operates in the same sphere as, it’s the repetitions and the ways those loops and riffs begin to engage with each other that brings out the real moments of beauty. And the push-and-pull between the album’s digitally layered repetitions and its moments of spontaneous improvisational flourishes –expressed maybe most spectacularly in “Swung From The Gutters” — is what keeps it unpredictable, that vital dynamic of luring, lulling, then shaking you into another space entirely done with a real liveliness even in the quiet moments.

But the end result, frustratingly enough, became the kind of album that earned praise and baffled ridicule in equally prominent measure. Without stars, lyrics, or convenient pop-cultural narratives to focus on, some critics merely found themselves transported to tedious dinner parties or middlebrow boutiques they expected to hear this music piped into. Whichever assessment’s less charitable — Spin calling it sexless and undanceable (“the results sound micromanaged, never congealing into the propulsive booty-call that’s lurking in their recessive genes… prisoners of their own good taste and tight pants”), or the NME reducing it to overpriced conversation pieces built by stuffy egotists (“a passionless vacuum, forever swallowing up and homogenising musical sub-genres, before spitting them back out in neat six-minute packages”) — it stuck enough that the band seemed put in a fighting-from-beneath position. Within a few years, the press decided that Rock Was Back, and despite the jittery brlliance of 2001’s Standards, the bulk of Tortoise’s ensuing decade was spent trying to figure out a place in an indie world that considered them too emotionless and stuffy before proceeding to lift a bunch of their ideas wholesale.

A year after the release of TNT, McEntire felt compelled to defend the album to The AV Club: “I mean, it’s not an easy record to just sit down and listen to. I’m certainly not trying to blow my own horn or anything, but I think it demands a certain amount of attention. There’s a lot going on within it. I think another thing that put so many people off is that it’s so long and there’s so much detail in there. There’s a lot to take in one sitting.” But then, it’s clear that the album’s nuances work so well once you’ve put in the time to let TNT insinuate itself into your mind — time, anyways, but no need for the effort. Attentive listening can work on its own level, especially if you want to appreciate the sheer craft of the record. But listen passively, and those sounds will find you soon enough, catching you off-guard and letting you know there’s a far bigger world of music that just rock can’t contain on its own. This album will take you there, and a lot of other places after that.