If the masc. [masculine] is the vertic. [vertical] line, then a man will recognize this element in the rising line of a forest; in the horizont. [horizontal] lines of the sea he will see his complement. Woman, with the horizont. line as element, sees herself in the recumbent lines of the sea, and her complement in the vert. lines of the forest.
Notes from Wikiquote
note in his sketchbook, undated but c. 1912; as quoted in Mondrian, - The Art of Destruction, Carel Blotkamp, Reaktion Books LTD. London 2001, p. 70