The British Constructivist Marlow Moss (1889–1958) was a pioneering modernist artist who also inspired her better-known colleagues such as Mondrian.The so-called “double line” was one of her most important inventions, which she began employing in her paintings in 1930, and which both Mondrian and Jean Gorin adopted for their own neo-sculptural works. Although neither artist overtly cited the influence, their works are nonetheless closely related to those of Moss, and they shared and admired her ideas about art and composition. A founder member of the Abstraction-Création group (alongside van Doesburg, Hélion and Vantongerloo), Moss also applied her linear structures and mathematical principles to reliefs and sculptures. These aspects of her work, along with her unusual life as a transgendered man, are discussed in this volume, by far the most substantial publication on her accomplishments.