The poster is one of Wes Wilson's early masterpieces for Bill Graham, and at its center is a face everyone in the room would have known: Grace Slick, photographed by Herb Greene, ringed by Wilson's tightening, hand-drawn letters. It advertised a weekend at the Fillmore Auditorium — Friday August 26 and Saturday August 27, 1966 — headlined by the 13th Floor Elevators, the acid-rock band out of Austin, Texas, with the Great Society second and Sopwith Camel third. The sheet says so plainly, in blue on red, and the credit line at the bottom reads "Photo by Herb Greene."

Almost none of that is what happened on stage.

The 13th Floor Elevators — the band the poster promises, the band that gives BG-25 its name in every catalog — did not play. The Fillmore Auditorium listings kept by Ross Hannan and Corry Arnold are unusually explicit about it: for August 27 they record "Country Joe & The Fish (replacing the Thirteenth Floor Elevators), Sopwith Camel," and then spell out the whole substitution — "Thirteenth Floor Elevators scheduled but did not play - replaced by Country Joe and the Fish, Sopwith Camel replaced Great Society." The night before, Friday the 26th, Quicksilver Messenger Service had stood in for the Elevators alongside the Great Society. Two nights, two different replacement headliners, and the band on the poster never appeared at all.

So the bill that actually played on Saturday, August 27, 1966 was Country Joe & the Fish and Sopwith Camel — a Berkeley jug-band-turned-electric-protest-outfit and a San Francisco pop group, standing in the space a Texas band had been drawn into.

There is a small, sharp irony sitting on the poster itself. The face at the center belongs to Grace Slick, then the singer of the Great Society — the act that got bumped off Saturday's bill by Sopwith Camel. Within a few months she would leave the Great Society for Jefferson Airplane and become one of the defining voices of the entire scene; Herb Greene's photograph of her here predates all of that. BG-25 is, in other words, a poster that mis-advertises its headliner, bumps the band whose singer it pictures, and accidentally immortalizes a musician on her way to a different group.

Down the street at the Avalon that same weekend, the Family Dog was closing out its own two-nighter: Captain Beefheart & His Magic Band with the Charlatans, on FD-23, running August 26 and 27. Beefheart at the Avalon and a phantom Elevators bill at the Fillmore — two houses, one weekend, and the poster art already doing what it would do for the next three years: outlast the night it was printed for.

A cataloguing note worth recording, because it took three archives to settle. Some Family Dog indexes file FD-73 against this August weekend; that is a misfile. FD-73 belongs to a run in late July 1967, and the Avalon poster for the corresponding weekend a year later — August 24–27, 1967 — is FD-77, Robert Fried's sheet for Big Brother & the Holding Company with Bo Diddley, Bukka White and the Salvation Army Banned. The correction matters because it is the difference between a poster and the wrong poster, which is the whole business this chronicle is in.

The lesson BG-25 teaches is the one we keep relearning. The catalog remembers the band on the sheet. The sheet remembers a show that never took place as drawn. Only the venue lists, kept by the people who were counting heads at the door, remember who actually walked out on stage.