[the double line in his paintings] is still one line, as in the case of your grooves [the wide sunken lines in the relief's, artist Gorin made that time]... In my last things the double line widens to form a plane, and yet it remains a line. Be that as it may, I believe that this question is one of those which lie beyond the realm of theory, and which are of such subtlety that they are rooted in the mystery of 'art'. But all that is not yet clear in my mind.

In a letter to the artist Gorin, [who stated that the double line broke the necessary symmetry], 31 January, 1934; as quoted in Mondrian, - The Art of Destruction, Carel Blotkamp, Reaktion Books LTD. London 2001, p. 215