The other element in the painter's work, line, also played a significant part in the examination and reflection of the abstract artists in the first two decades of the twentieth century. To be sure, in purely quantitative terms line seems to have attracted less of their attention than color. Thus, while in On theSpiritual in Art Kandinsky devoted a whole chapter specifically to the "Effects of Color:' he treated line mainly in a chapter called "The Language of Forms and Colors:' Even in this chapter the main parts were, in fact, devoted to color rather than to line. As we shall see, a similar situation also obtains in the theoretical texts composed by Mondrian at this period. Evidently, the founders of abstract painting considered color as the more interesting, and in their specific context also the more important, feature of the two. Yetwhile they may in fact have devoted more intellectual effort to color, line was and remained a crucial problem for them. Understanding line was of the utmost urgency in the formulation of a theory of abstract painting. In this chapter I shall try to show that in fact line, rather than color, was a touchstone for the validity of "abstraction:' as they understood it, in painting.