Later she began making irregular holes in the canvases ("apertures", as she called them: the earliest example is "Winter Windows", 1966–1967), disclosing deeper layers of painted canvas underneath (so-called "double canvases" - and sometimes triple canvases), with painted-on "false shadows", etc. - increasingly invoking the third dimension, creating tactile, sculptural effects while remaining within the convention of the framed, rectangular oil painting. The apertures also suggest a view into some sort of psychological interior, as though the second canvas - seen only partially, through the hole in the forward canvas - were some half-concealed secret, perhaps even another whole painting that we will never see.