A simple protractor helped artist Frank Stella create the perfect semicircular forms in his aptly named Protractor series of paintings. Stella used the instrument to draw small-scale versions of his designs before transferring them to much larger canvases, using a compass-like device he fashioned himself.

He began this series in 1967, creating three different design variations on 31 canvas formats (for 93 paintings total), and named many of them after the circular cities he visited in the Middle East in 1963.

One of these paintings, Hagmatana III (named after an archaeological mound containing the ruins of an ancient city on the site of present-day Hamedan, Iran), is part of the permanent collection at The Glass House, a National Trust Historic Site in New Canaan, Connecticut. The site’s original owner and architect, Philip Johnson, was a longtime friend of the artist and a supporter of his work. Today there are 18 paintings, sculptures, and multimedia works by Stella at The Glass House.