Diptych

One form 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.

If the hinge of the diptych is level with the ground, and both dials show the same time, the dials are showing the mean solar time, the hinge faces north (in the northern hemisphere), and the gnomon is parallel with the axis of rotation of the Earth. At noon, there is no difference, but at sunset or sunrise, each degree of error in the gnomon's adjustment to latitude creates a difference of eight minutes in the two time readings (a degree is 1/15 of a solar hour).

Some diptychs had compass roses (to measure bearings to geographic features) and latitude measurement bobs. Some authorities believe that large versions (a meter or more in width) were used for maritime navigation, and thereby came to acquire an air of magic in the popular mind.

Of course, all these functions could be combined in one pocket-sized artifact. It could be a very convenient thing to keep in one's pocket.