Diptych sundial

During the middle ages advanced yet portable astronomical instruments were developed: the astrolabe whose functions included that of a sundial.
The most successful portable sundial was called a diptych. It consisted of two small flat faces, joined by a hinge. The gnomon was a string between the two faces. When the string was tight, the two faces formed both a vertical and horizontal sundial. The best material was white ivory, inlaid with black lacquer markings. The best gnomons were black braided silk or linen.