The Blues Project's Matrix week ran on: Sunday, September 4, 1966, still with Robbie Basho.

It is worth explaining what the room actually was, because the Matrix is named constantly in accounts of this era and almost never described. It stood at 3138 Fillmore Street in Cow Hollow, in a converted beer-and-pizza shop of roughly a hundred capacity — about forty feet by eighty, the ceiling around ten feet at the front and rising to perhaps twenty further back. Marty Balin opened it on August 13, 1965 after persuading three limited partners to put in three thousand dollars apiece for seventy-five percent of the business, keeping a quarter for himself in exchange for creating and running it. He installed the band he had just assembled as the house band, and on opening night they settled on the name Jefferson Airplane. Ralph J. Gleason saw them there early and became their champion in the *Chronicle*.

So the Matrix is not a minor footnote to the ballrooms — it is where the scene's first band was built, a year before the Fillmore had a poster series.

Four nights into a six-night stand, in a room holding a couple of hundred at the most, while four blocks away the Grateful Dead headlined the Fillmore for the first time. The Matrix's booking pattern — one act, most of a week, no poster — is the opposite of the ballrooms', and between them the two models cover most of what live music in San Francisco actually was in 1966.