Editing Sundial (section)

==Multiple dials==
Sundials are sometimes combined into multiple dials. If two or more dials that operate on different principles — such as an [[Analemmatic sundial|analemmatic dial]] and a [[#Horizontal_sundials|horizontal]] or [[#Vertical_sundials|vertical]] dial — are combined, the resulting multiple dial becomes self-aligning, most of the time. Both dials need to output both time and declination. In other words, the direction of [[true north]] need not be determined; the dials are oriented correctly when they read the same time and declination. However, the most common forms combine dials are based on the same principle and the analemmatic does not normally output the declination of the sun, thus are not self-aligning.<ref>{{cite web|last1=Bailey|first1=Roger|title=1 Conference Retrospective: Victoria BC 2015|url=http://www.sundials.org/attachments/article/289/2015%20NASS%20Conference%20Victoria.pdf |website=NASS Conferences|publisher=North American Sundial Society|access-date=4 December 2015|archive-date=8 December 2015|archive-url=https://web.archive.org/web/20151208144915/http://www.sundials.org/attachments/article/289/2015%20NASS%20Conference%20Victoria.pdf|url-status=live}}</ref>

===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>

===Multiface dials===

A common type of multiple dial has sundials on every face of a [[Platonic solid]] (regular polyhedron), usually a [[cube]].<ref>{{harvp|Rohr|1965|p=118}}; {{harvp|Waugh|1973|pp=155–156}}; {{harvp|Mayall|Mayall|1994|p=59}}</ref>

Extremely ornate sundials can be composed in this way, by applying a sundial to every surface of a solid object.

In some cases, the sundials are formed as hollows in a solid object, e.g., a cylindrical hollow aligned with the Earth's rotational axis (in which the edges play the role of styles) or a spherical hollow in the ancient tradition of the ''hemisphaerium'' or the ''antiboreum''. (See the History section above.) In some cases, these multiface dials are small enough to sit on a desk, whereas in others, they are large stone monuments.

A Polyhedral's dial faces can be designed to give the time for different time-zones simultaneously. Examples include the [[Scottish sundial]] of the 17th and 18th&nbsp;centuries, which was often an extremely complex shape of polyhedral, and even convex faces.

===Prismatic dials===
Prismatic dials are a special case of polar dials, in which the sharp edges of a [[prism (geometry)|prism]] of a concave [[polygon]] serve as the styles and the sides of the prism receive the shadow.<ref>{{harvp|Waugh|1973|pp= 181–190}}</ref> Examples include a three-dimensional cross or star of David on gravestones.