Historically, there were two camps, one suggesting that Marlow Moss thought of the double line first and introduced the idea to Mondrian; the other (the Mondrian Establishment), dismissing the notion that Moss, a mere Mondrian imitator, could possibly have been that inventive or have had that much influence over Mondrian and his subsequent works.
On an early MM page, I said, "I notice that Mondrian scholar, Prof. Yve-Alain Bois is dismissive of MM's influence on PM",
There is a myth, circulated in part by Vantongerloo's correspondence with Gorin, to the effect that Mondrian borrowed the double line from a young English painter, Marlow Moss, who saw herself as a neo-plasticist. A few months before Mondrian adopted this new element in his art, she did in fact paint (an may have exhibited) double-line pictures; but one need only glance at Moss' earlier "double-line" pictures to realize that she makes very different use of this element: her couples lines are so unequal in thickness that they can be read neither as forming a single linear entity, nor as belonging to the same plane. See, for example, the two "double-line" compositions by Moss, dated 1931, and reproduced in the first issue of Abstraction-Création, dated 1932 (p. 26). It is only in the second issue of this journal (dated 1933) that one can see a "double-line" by Moss that has something in common with those of Mondrian (p. 29); the same issue carries a reproduction of Mondrian's first "double-line" canvas (p. 31). In other words, Mondrian does not first criticize, then adopt, Moss' invention, as Vantongerloo suggests: he is at first a skeptic, then understands his lack of interest in Moss' version, then demonstrates how, and for what destructive end, the double line could be used in neo-plastic art.
Yve-Alain Bois in note 156 to an essay, The Iconoclast, in Piet Mondrian 1872-1944, Bullfinch Press, 1994, p. 371
I continued, "Ms Moss should not feel too hurt or isolated by this treatment, Prof. Bois is dismissive of numerous people and publications in this essay".
It was in this context that I first encounterd Marlow Moss in around 2002 in a publication by Robert Welsh, Bulletin 29/1977 from the National Gallery of Canada. I wrote that he "considers the origin of Double Lines in Mondrian's work. One interesting possibility is that PM was helped towards the idea by the English constructivist painter Marlow Moss (1890-1958)" I quoted his footnote,
A.H. Nijhoff (Introduction, ex. cat. Marlow Moss; Amsterdam: Stedelijk Museum, 1962, n.p.), a close personal friend of Moss, relates how upon first publicly exhibiting a double-line painting, circa 1930-1931, the artist received a written request for an explanation from Mondrian. Her illustrated reply cited three basic reasons: (1) single lines produce an impression of planar surfaces; (2) single lines render the composition static; and (3). double, or multiple, lines have a dynamic effect by ensuring "a continuity of related and interrelated rhythm in space." Such reasoning certainly would have appealed to Mondrian, whether or not one considers Moss the principal stimulus for Mondrian's adoption in 1932 of the double-line convention … although Composition with Yellow Lines is dated 1933, according to documentation kindly supplied by H. Henkels of the Hague Gemeentemuseum, it was commissioned and presumably begun the previous year, which allows it to be considered the final major "single-line" painting.
Nowadays, it is generally accepted that Moss thought of it; introduced to Mondrian; he asked her why she used it; she responded with her objective reasons; he couldn't understand these but just adopted the idea subjectively. See card, fig. 3: the subject of this pack of cards, Mondrian's women is at first patronising and echoes this whole argument, but the title, love and inspiration (fig. 4), does ameliorate this somewhat.
This originality debate, instigated by Vantongerloo and subsequently played out in writing on Moss and Mondrian to date, becomes somewhat facile. What can be a fruitful subject for analysis is the works themselves.