Original 1st printing 12&3/8 x 19&7/8" concert/event poster for the Grateful Dead, Big Brother & the Holding Company (featuring Janis Joplin) and Quicksilver Messenger Service appearing together in a benefit performance (for the San Francisco Zen Mountain Center), held at the Avalon Ballroom in San Francisco, CA on 11/13/1966.
Designer unknown (photo by Robert Bono), image ("Zenefit") is featured in the Art of Rock book on page 184 (plate 2.181).
Medium-thick flat vellum stock poster is in very good (B/B+) condition; there is a semi-flattened horizontal center crease, staple/pin holes in all four corners, light handling wear, a few edge dings and a thin horizontal bend traversing the upper margin, but most of the conditional issues are very well camouflaged by the design.
A sweet example of this VERY early Grateful Dead/Janis benefit poster from the Avalon.