Editing Diptych (section)

==Diptych sundial==
{{Main|Sundial#Diptych (tablet) sundial}}
[[File:Sundial in the form of a mandolin - Project Gutenberg eText 15050.png|thumb|Diptych sundial in the form of a [[lute]], {{Circa|1612}}]]

A 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.<ref>{{citation|title= Sundials:Their Theory and Construction|author= Albert E. Waugh|year= 1973|publisher= Dover|isbn= 0-486-22947-5|url-access= registration|url= https://archive.org/details/sundialstheirthe00albe}}</ref> This property of self alignment is only true for diptychs historically in the case for a combination of an [[analemmatic sundial|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.<ref>{{citation|title= Sundials:History Theory and Practice|author= Rene J. Rohr|year= 1996|publisher= Dover|isbn= 0-486-29139-1|url-access= registration|url= https://archive.org/details/sundialshistoryt0000rohr}}</ref>

Some diptychs had rough calendars, in the form of [[pelekinons]] calibrated to a [[redirection problem?|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.