The Fillmore concert posters of 1966–1971 are the visual fingerprint of San Francisco's acid counterculture. Stanley Mouse, Alton Kelley, Rick Griffin, Wes Wilson, and Victor Moscoso — the "Big Five" — invented a graphic language of vibrating complementary colors, liquid lettering melted past legibility, and swirling mandala compositions designed to simulate altered perception on paper.

Printed as cheap handbills for Grateful Dead and Jimi Hendrix shows at Bill Graham's Fillmore Auditorium, these posters were deliberately tribal — only the initiated could decode them. Their aesthetic proves that design can be a gatekeeping ritual: if you have to ask what it says, you weren't there.