Also in the 1960s, Stella began to use a wider range of colors, typically arranged in straight or curved lines. Later he began his "Protractor Series" (71) of paintings, in which arcs, sometimes overlapping, within square borders are arranged side by side to produce full and half circles painted in rings of concentric color. These paintings are named after circular cities he had visited while in the Middle East earlier in the 1960s. The Irregular Polygon canvases and Protractor series further extended the concept of the shaped canvas.