CHICAGO — At age 18, Scott Ligon spent his nights getting paid to play guitar and his days shopping for records. It was 1988, and he was one of the rising regulars of the St. Louis bar-band scene. He knew his blues, his rock-and-roll and had days’ worth of country and soul covers in his head. One day his older brother brought home a new record as usual, but this time Ligon didn’t recognize the band. It was NRBQ’s “God Bless Us All,” a live album released the year before, and it took only to the middle of the second song, “Here Comes Terry,” before he felt his brain shift.

“The bridge is just a beautiful piece of composition,” he explained to me recently by phone. “I’d heard a lot of music at that point, and this was something I’d never heard before.”

He swiftly tracked down the band’s earlier albums, nearly 20 years’ worth at that point. “It felt like I was listening to the Beatles,” Ligon said. “That’s how good they were. I saw them live and something happened to me. I had this vision, like, ‘I’m supposed to be in this band.’ ”

For the next 20 years, Ligon says, NRBQ “haunted” him. He moved to Chicago and became one of that city’s go-to talents for studio and live guitar work. But his NRBQ dreams were a running joke among his broadening circle of friends. “Anyone who knew me knew I felt destined,” he says.

In late June, many of those friends gathered for NRBQ’s “Midwestern Conference,” a two-night stint at Chicago’s venerable independent social hall, the Hideout. And up front, stage left as he has been since 2011, his dream finally fulfilled, was Scott Ligon. He’s the second-longest-tenured member of the band after founding keyboardist Terry Adams, who, now in his early 70s, has been hacking an inimitable, zigzagging path through the outskirts of pop music since the Johnson administration. These days, the band’s three other members all live and make music in Chicago, but it takes Adams to make something an NRBQ show. Wherever Adams goes, the air seems to shift around him.

For the Midwestern Conference, the band had supplied branded name tags for the crowd to get to know one another, but the gesture wasn’t necessary. The Chicagoland NRBQ faithfuls arrived early and thirsty, and a ­family-reunion vibe pervaded. They filled the Hideout’s cozy patio, in equal numbers of men and women. More than a few wore NRBQ T-shirts. The crowd skewed older but felt youthful and familiar. The merch table did fleet business, and the wood-paneled main room was packed and loud well before set time.

I noticed one woman greeting old friends on the patio. She seemed to be keeping a roll call, and new people kept strolling in to join the group. I asked if it was a fan club.

“I think for this band,” she said, “anyone who knows them is pretty much the fan club.”

NRBQ has never had a hit record, and they’ve never been played much on the radio or listed in magazine polls of great albums or players. But for more than 50 years, they have conjured a slippery blend of early R&B, hardcore country, novelty songs and rockabilly by seizing the improvisational energy of jazz.

“A day we play is a holy day,” Adams said in an interview a few weeks after the Chicago show. “You wake up somewhere and have that first cup of espresso, hear a song in the coffee shop. It’s all leading to that 10 p.m. showtime. The vibrations in the room feel different every night. We approach every note as how we’re feeling right then. That sounds too churchlike, but it’s true.”

When he first heard and saw NRBQ, Ligon says, he recognized “these guys are having more fun than anybody. But they’re doing it in a sophisticated way. That version of the band was like a living cartoon.”

He’s referring to what most fans would call the “classic” NRBQ, which lasted from 1974 to 1994: Adams, of course, with Joey Spampinato on bass, Tom Ardolino on drums and “Big Al” Anderson on guitar. Spampinato had been onboard since the debut, wrote great pop songs and possessed a beautiful R&B tenor. Ardolino had the deepest swing and hardest backbeat on earth, and regularly interacted onstage with a puppet of himself, “Little Tommy.” And Big Al was simply one of the most versatile guitarists imaginable, capable of country twang or squalls of feedback; he also penned the gorgeous “Ridin’ in My Car,” which might be the group’s best-known original song.

This is the group that won the adoration of Bonnie Raitt, who recorded two of their songs for the 1982 record “Green Light,” as well as R.E.M., who brought them on the road in 1989. Keith Richards enlisted Spampinato on bass for Chuck Berry’s backing band in the film “Hail! Hail! Rock ‘n’ Roll.” The classic NRBQ connected musical dots that no one else had thought to before. They made records and played concerts with Skeeter Davis, Carl Perkins, the Lovin’ Spoonful’s John Sebastian, members of the Sun Ra Arkestra and the wrestler Captain Lou Albano, who served as their manager in the 1980s.

Reached by phone, Raitt was thrilled for the chance to tell one more person about “one of my favorite bands.”

“I would put them on the level of the Beatles,” she said at one point, and the rest of our conversation was set at a similar pitch. She first heard NRBQ playing with Perkins in the mid-’70s and had the requisite mind-blowing experience. “When I became a headliner, as often as their schedules permitted, I put them on the show and we double-billed for many decades. They play the roots of rock-and-roll the way it should be played. They absolutely nailed the best part of what made blues, R&B, and rock-and-roll cross paths. They are without parallel.”

Anderson left the group and became an ace songwriter in Nashville, and Spampinato’s brother Johnny took over guitar until a hiatus in 2006. In all, more than a dozen full-time members have played in NRBQ over the years, and that doesn’t include the Whole Wheat Horns, the group’s occasional sidemen, who have rotated in and out through the years as well.

But, Adams insists, “no one has ever replaced someone else, and no one is ever the model role for their instrument. My first partner was Steve Ferguson. We played in Louisville and in Florida. Split for a year, picked back up in 1968. When he left, there was no replacing Steve. But someone else will come in. And that’s what changes the band, the personality of those involved.”

The 2006 hiatus was brought on as Adams recovered from throat cancer. Ligon swears that he was finally over his obsession by this point, but then one of his bands opened for Adams and the impossible happened — they had an instant connection. The Terry Adams Rock & Roll Quartet formed in 2007, featuring Ligon on guitar, and in 2011, Adams decided to revive the classic name.

At the Hideout, Adams and Ligon flanked the stage, with bassist Casey McDonough and drummer John Perrin between them and a three-piece Whole Wheat Horns on a riser hanging off stage right. Adams and Ligon are both tall, blond and very aware of the audience when onstage — they strut and smile and shout to their bandmates, and harmonize perfectly with McDonough at maximum volume all night. Adams’s Easter-grass hair blew around him and his mouth hung open, snapping occasionally into a cavernous grin as he stabbed aggressive block chords on the piano like his hero, Thelonious Monk.

At this point, NRBQ is like an extension of Adams’s sensibilities, a conduit for his every musical whim. The band is simultaneously meticulous and goofy: Ligon’s solos are dynamic; Perrin and McDonough navigate every style and curlicue arrangement with ease. But there’s no set list, only the directions and decisions offered by Adams.

In Chicago, they covered Duke Ellington, Clarence “Frogman” Henry, Stevie Wonder and the progressive big-band pioneer Stan Kenton, among others. They played “Magnet,” a clever, moony love song that Adams wrote with Spampinato in 1972, and they played “Love This Love We Got,” a clever, moony love song that Adams wrote with Ligon and McDonough in 2014. More than two hours they went, including two encores.

“Terry takes the responsibility for people having a good time or not,” Ligon says. “It means everything to him. He’s trying to keep things interesting, to show every band member in their best light. Everything is about what it feels like right now. What’s the best song for this moment? He wants to show that life can be funny, we can be silly. He feels really deeply for people.”

Adams’s affection for Monk took root when he was a rabid, aspiring musical savant himself. He routinely raided the record stores in his hometown of Louisville and picked one of Monk’s early records off the wall at random. Adams became a regular attendee at Monk’s shows, enough that he met the pianist multiple times and was granted a permanent spot on the guest list.

His pop credentials notwithstanding, Adams has had a significant career in jazz. He’s played in the Sun Ra Arkestra and as a duo with that legendary group’s longtime leader, Marshall Allen. He backed legendary vocalist Annie Ross on-screen in the 1993 Robert Altman movie “Short Cuts” and has released a stream of instrumental records.

For now, NRBQ is Adams’s sole performance and musical outlet. The 1969 debut was remastered on CD and vinyl last year, and a 50-year box set came recently as well. But when we spoke, Adams was focused on the music they’re recording in August.

“It’s hard to encapsulate how brilliant I think he is,” Raitt says. “He’s got big open ears and incredible intelligence. But he’s also extremely childlike and full of wonder and playfulness. He’s got extensions into other atmospheres that the rest of us can’t pick up on. I know that sounds a little woo-woo.”

Perhaps, but Terry Adams is not averse to woo-woo, and that might be the best explanation for his grand loopy vision, combining so many strands of American music, or the fanatic adoration it inspires.

“I’m sitting here at a park, looking at a lot of beautiful trees,” he said. “Life is beautiful. To have the gift of putting that into sound, I feel blessed. I’m one of the few people who can say, I’ve never had a job, I’ve never even played a song I didn’t want to. When I’m playing, I think about it. I look up and I think, ‘I’m doing this still.’ ”