Alfred Stieglitz's Manhattan gallery was called 291.
The gallery took its name from its address at 291 Fifth Avenue in New York City. It was founded by Stieglitz and Edward Steichen as the Little Galleries of the Photo-Secession, then became widely known simply as 291.
Stieglitz used the space to exhibit photography and European modern art alongside work by American artists. In 1916, he showed O'Keeffe's abstract charcoal drawings there after a friend sent them to him without her initial knowledge. The exhibition helped launch her public career in New York.
291 closed in 1917, but Stieglitz later operated other galleries, including the Intimate Gallery and An American Place. Those later venues are sometimes mistakenly given as the place where O'Keeffe first appeared. The address 291 is the specific gallery name tied to her breakthrough exhibition and to Stieglitz's early promotion of her work.