The press list for the perennially top-selling 9:30 Club’s star-studded 30th anniversary show Monday night was too “tight” for even Arts Desk. Luckily, YouTube was all over it: see Dave Grohl with Scream, Teo Leo, the Fleshtones, the Pietasters.

“If these bands didn’t like this place, they wouldn’t show up,” noted host Henry Rollins. “It is the holiday. Many of them have other places to go.” Rollins himself had a heckuva time making Monday’s big event. “It took two airplanes to get here, I’m very, very sorry,” he explained his tardiness to the packed crowd. “I cannot tell you how much it guts me to find out that I missed the Evens, how much it guts me to find out I missed the Slickee Boys.”

Rollins fondly recalled the good old days at the tiny original 9:30 Club at 930 F St. NW, before the big move to its current cavernous digs on V Street. “Many of you ancient fans have memories of those great days and that odd stage with that pillar right in the middle, where the band would lose you, you’d lose the band, try to find them….”

My personal favorite 9:30 story? The one about how it’s always a full house—even when it’s half-empty (“Crowd Control,” Show & Tell, 08/27/2004):

That’s because “capacity” is a relative term at the 9:30, which can hold up to 1,200 concertgoers. But whereas most clubs leave it to the audience to fill the available floor space, the 9:30 tailors its floor space to the available audience.

That’s the benefit of having a stage on wheels—a concept developed during the club’s transition from its more intimate digs on F Street NW to its current industry-leading location back in 1996.

“We had this niche at the old club of being the coolest little place for bands to play when they first started,” says promoter Seth Hurwitz of Bethesda-based I.M.P. Productions. “We knew if we went and got this enormous new place that small bands wouldn’t want to play there….So we had to have a place that also worked on a small capacity.”

Enter Chad Housekneckt, I.M.P.’s “director of ambience and atmosphere,” who proposed a solution: “Why don’t we just move the stage back and forth?”

On nights when promoters expect less than a full house, the stage crew rolls the whole performance platform forward. And voilà: What might have seemed half-empty will now look full, and all that excess space is covered up behind a big black curtain.

“Usually, it doesn’t look so good if there’s only 800 people in a 1,200-capacity room,” says Kevin French, manager for D.C. pop-punk outfit Washington Social Club, among others. “But at 9:30 Club, it can look great.”

And French should know: Self-described “peculiarly styled pop” group the Decemberists, which he also manages, drew just 665 for its 9:30 show on June 14. But it looked crowded. The attendance 9:30 reported to Pollstar: “100%.”

Hurwitz says the fuzzy math is all about keeping bands and their agents happy—and thereby keeping those bands coming back to the 9:30.

“If you reported that a band did 400 people in a 1,200-person room, then that band would consider that a failure,” Hurwitz says, “and they wouldn’t want to play there anymore.”