Agnes Martin's paintings of austere graphite grids and subtle bands of color grew out of a pressing need to express what the artist called "the most simple, powerful things." Her breakthrough came in the mid-1960s with a series of simple pencil grids ruled directly onto gessoed canvases, of which the Modern's "Leaf," 1965, is a classic example. Throughout the 1970s, Martin began opening up her grids, emphasizing the intervals between the lines and creating different sensations of space and atmos