Editing Sundial (section)

===Diptych (tablet) sundial===
[[File:Sundial in the form of a mandolin - Project Gutenberg eText 15050.png|thumb|170px|upright|Diptych sundial in the form of a [[lute]], {{circa|1612}}. The gnomons-style is a string stretched between a horizontal and vertical face. This sundial also has a small nodus (a bead on the string) that tells time on the hyperbolic ''pelikinon'', just above the date on the vertical face.]]

The '''[[diptych]]''' consisted of two small flat faces, joined by a hinge.<ref>{{harvp|Rohr|1965|p=112}}; {{harvp|Waugh|1973|pp=154–155}}; {{harvp|Mayall|Mayall|1994|pp=23–24}}</ref> Diptychs usually folded into little flat boxes suitable for a pocket. The gnomon was a string between the two faces. When the string was tight, the two faces formed both a vertical and horizontal sundial. These were made of white ivory, inlaid with black lacquer markings. The gnomons were black braided silk, linen or hemp string. With a knot or bead on the string as a nodus, and the correct markings, a diptych (really any sundial large enough) can keep a calendar well-enough to plant crops. A common error describes the diptych dial as self-aligning. This is not correct for diptych dials consisting of a horizontal and vertical dial using a string gnomon between faces, no matter the orientation of the dial faces. Since the string gnomon is continuous, the shadows must meet at the hinge; hence, ''any'' orientation of the dial will show the same time on both dials.<ref>{{harvp|Waugh|1973|p=155}}</ref>