“Punk” is impossible to define unambiguously. The same can’t be said for “progressive rock.” The genre aspires to the complexity of classical music or jazz, and it operates according to fairly rigid standards: It eschews verse-chorus-bridge structures, includes non-rock instrumentation, uses complex meters, and de-emphasizes traditional chord progressions and scales in favor of more florid, sometimes meandering movements.

Fantasy-based lyrics are common, too, so the idyllic Storybook Farm on the fringe of Chapel Hill was the perfect setting for ProgDay, a weekend festival that just finished its 13th year. A gravel drive wended amid greenhouses, gardens, ponds, wooden fences, birdhouses and pastures. Rusted trucks and tumbledown shacks shared space with festival-goers’ gleaming Volvos and Scions. Cricket song vied with protracted guitar solos.

ProgDay was founded by Peter Renfro in 1995, making it the longest-running prog festival in the world. Since 2001, it’s been run by a volunteer committee (this year under the direction of festival coordinator Steve Sly) that handles financing, promotions and booking.

Since prog has virtually no mainstream foothold, such festivals are key for bringing bands and the fans who enjoy them together. This year’s ProgDay drew bands from as far away as Japan and Sweden, whose governments underwrite their travel. According to ProgDay promotions chairman Ron Ayersa 60-year-old from Roanoke, Va., who’s been into prog since the 1970s heyday of King Crimson, Genesis, ELP, Jethro Tull and Yesa prog band who plays to several hundred fans at the Chapel Hill festival might play to 25 in New York City, most of whom found out about the band from the festival circuit.

But the strictures of prog, something of a purist’s genre, don’t mean there’s no diversity: Instead, on Saturday, Japan’s Naikaku was a metal band in tattered, blood-stained scrubs with a female flute lead in a kimono, while New York’s Frogg Café mixed rock instrumentation with sax and violin. They were mellower and groovier than Naikaku, almost like a jam banda genre closely related to prog, although according to Ayers, “the twain have yet to really meet.”

ProgDay felt very intimate for a festival, with a patronage roughly equal to a half-capacity crowd at the Cat’s Cradle. The stage sat low, under a wooden pavilion, with no barricades between it and the spectators. The audience included men and women, although it skewed middle-aged male, and was almost entirely Caucasian. Most fans brought their own brightly colored umbrellas, awnings, tents, camping chairs and coolers.

Like all genres, prog is subject to certain stereotypes, and just as indie rock shows feature a surfeit of studded belts and tight jeans, ProgDay had its share of graying ponytails and vintage Rush tour T-shirts. But little of prog’s vaunted snobbishness was apparent: Festival-goers were friendly and easy-going. Since prog offers little by way of financial or cultural rewards, its adherents are unquestionably in it for no other reason than their love for sounds with more musicological depth than pop offers.

Buster Harvey, a 45-year-old from the D.C. area, attended the festival with his family, wearing homemade T-shirts that read “Prog Dad,” “Prog Mom” and “Prog Kid.” Harvey got into prog at boarding school in the 1970s: An older kid on his hall would play bands like Yes, and Harvey would sit outside his door and “just groove on the stuff.” Now he’s passing on this love to his kids, who he says look forward to the festival all summer, ensuring that this self-sustaining network of esoteric music fans (“kind of like a cult,” according to Ayers) will secretly thrive for years to come, a stable stone in the rushing river of mainstream trends.