The first Agnes Martin retrospective after the artist's death opened at Tate Modern in London, June 3 - October 11, 2015. In advance of the exhibition, the museum's member magazine, Tate Etc., published my article on Martin -- as well as a short reminiscence by the late artist Rosemarie Santoro -- together under the title, "Square Dance of Joy." Republished online at Momus, a Canadian magazine of contemporary art criticism, as "On the Subtle Joy of Agnes Martin."