face was on the inside of each leaf. One leaf formed a vertical sundial, the other a horizontal sundial. The shadow caster, or gnomon was a string between them, and calibrated as to how far they should open, as the angle is critical. Such a sundial can be adjusted to any latitude by tilting it so its gnomon is parallel to the Earth's axis of rotation. A common error states that if both dials show the same time, the instrument is oriented correctly and faces north (in the northern hemisphere). A Diptych made as stated as a combined vertical and horizontal sundial with a string gnomon will show the same time on both dials regardless of orientation. This property of self alignment is only true for diptychs historically in the case for a combination of an analemmatic and a vertical sundial. A double dial on a flat plate consisting of a horizontal and an analemmatic dial will also be aligned properly if both dials show the same time. Some diptychs had rough calendars, in the form of pelekinons calibrated to a nodus in the form of a bead or knot on the string. These are accurate to about a week, which was good enough to time planting of crops. See also Wilton Diptych, an extremely rare survival of a late Medieval religious panel painting from England Triptych Polyptych References Bibliography Marco Cristini: Eburnei nuntii: i dittici consolari e la diplomazia imperiale del VI secolo. In: Historia: Zeitschrift für Alte Geschichte 68 (2019), pp. 489–520. Wolfgang Kermer: Studien zum Diptychon in der sakralen Malerei: von den Anfängen bis zur Mitte des sechzehnten Jahrhunderts: mit einem Katalog. Düsseldorf: Dr. Stehle, 1967 (Phil. Diss. Tübingen 1966) Ralf Kern: Wissenschaftliche Instrumente in ihrer Zeit. Vom 15. – 19. Jahrhundert. Verlag der Buchhandlung Walther König 2010, External links National Gallery of Art, Prayers and Portraits: Unfolding the Netherlandish Diptych (2006) Diptych sundials, National Maritime Museum Measuring instruments Ivory works of art Picture framing Altarpieces