A convenient form of pocket compass dial. It consists of a pair of rectangular metal or ivory plates, hinged along a shorter edge. When closed it is flat, but in use it is opened, with the upper plate vertical and the lower remaining horizontal. In this position the string gnomon, which passes through a hole in each plate, becomes taut. This casts a shadow on a dial marked on the horizontal plate, in which a compass is also set. The same gnomon often serves a vertical south dial on the under-surface of the upper plate. Diptych dials often have other 'furniture', the most common being small horizontal, vertical, or cup dials showing Italian and Babylonian hours, others being pin-gnomon dials showing unequal hours and the length of the daylight period. In some versions, a nocturnal or a volvelle for interconverting lunar and solar times may also be included.