The New York gallery called 291 helped introduce Georgia O'Keeffe's early work to the public.
The gallery was founded and directed by Alfred Stieglitz. Its formal name was the Little Galleries of the Photo-Secession, but it became widely known as 291 after its address at 291 Fifth Avenue in Manhattan.
Stieglitz exhibited O'Keeffe's abstract charcoal drawings there in 1916. The exhibition was arranged after Anita Pollitzer showed the drawings to him, and it marked O'Keeffe's entry into the New York art scene. O'Keeffe was teaching in Texas when the drawings were shown.
The gallery also displayed photography and European modern art, including works associated with Pablo Picasso, Henri Matisse, and Constantin Brâncuși. Stieglitz later ran other galleries, including the Intimate Gallery and An American Place, so those names should not be substituted for 291 in this question.