Ivory consular diptych of Areobindus, Byzantium, 506 AD, Louvre museum

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A diptych is any object with two flat plates attached at a hinge. Devices of this form were quite popular in the ancient world, types existing for recording notes and for measuring time and direction. The term is also used figuratively for a thematically-linked sequence of two books. more...

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Note: This article discusses diptyches in the first sense. For paintings arranged in such a way, see polyptych.

Traditional diptychs are boxwood, with stamped hour lines and lacquered or varnished finishes. Some were also ivory (superior because it is easiest to read and less prone to wear than wood), or metal (sturdy, harder to read but less expensive than ivory).

One form of diptych was like a shallow box. It had two wooden leaves with hollows on the inside edges, filled with wax, and space for a small wooden scriber. This permitted one to take waterproof notes in the wax without wasting money on paper. The wax could be smoothed and reused. It was probably excellent for shopping lists or other reminders.

The other form was a portable sundial. 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 how far open they should go as the angle is critical.

A sundial can be adjusted to any latitude by tilting it so its gnomon is parallel to the Earth's axis of rotation. However, the longitude is critical for an accurate local solar time, and is corrected by leveling the diptych on its axis from east to west.

If the hinge of the diptych is level with the ground (classically measured with a rolling marble in a slot), and both dials show the same time, the dials will show the apparent solar time, the hinge faces north (in the northern hemisphere), and the gnomon is parallel with the axis of rotation of the Earth.

Achieving all these functions is almost a lost art. A north-indicating diptych is possible only if the two sundials do not have the same complementary sun angle. The best real diptychs never consisted of two mirror-imaged 45 degree sundials; usually they were adjusted so that at the owner's latitude, the bottom leaf was level not just east-to-west, but north-to-south. That is, if the gnomon is not parallel to the earth's rotational axis, then since the two faces have different trigonometric projections, they will show different times. For example, if the gnomon deviates from the correct elevation angle at 9am or 3pm, each degree of error in the gnomon's elevation creates a difference of four minutes (one degree of angle) in the time readings of the two faces. However, at 6am, 6pm and noon, a deviation in the gnomon's elevation angle produces no change in times. Near noon, if the gnomon deviates from pointing north and south, the times of the two faces will deviate. At 6am and 6pm, deviations from north and south have no effect. Holding a diptych so that its gnomon-string is at the correct angle is often finicky, especially near sunrise, sunset and noon, so many later diptychs had magnetic compasses and plumb-bobs to help, but these were luxuries, not necessities.