The New York gallery was 291.
The gallery took its name from its address at 291 Fifth Avenue in Manhattan. Alfred Stieglitz operated it as an important venue for modern art and photography, and it became closely associated with the introduction of European avant-garde ideas to American audiences.
Anita Pollitzer brought some of Georgia O'Keeffe’s drawings to Stieglitz’s attention. He exhibited them at 291 in 1916, and O'Keeffe’s first solo exhibition there followed in 1917. This connection helped establish her in the New York art world and began the professional relationship that later became personal.
291 is sometimes confused with the later galleries Stieglitz operated, especially the Intimate Gallery and An American Place. Those were separate spaces from different periods. The gallery’s short numerical name is the key identifying fact: it came directly from the street address rather than from an artist or organization.