Epact Scientific Instruments of Medieval and Renaissance Europe

The reverse has a line on each arm with 2 sector scales whose positions are marked by dots. One pair run outward 20 to 2 by 1, non-linear increasing; the second set of dots are on the same lines and are circled to distinguish them, running outward 20 to 3 by 1, non-linear increasing. Where a dot serves both scales (6 / 19, and 2 / 6) it is marked by a circle with points, like a star.

Two subsidiary arms are stowed in the inner edges of the principal arms. They can be folded out and fixed together to form a geometrical quadrant, with an inner scale of degrees 0 to 90, divided to 5, subdivided to 1, numbered by 5, and an outer geometrical quadrant scale 0 to 300 to 0, divided to 5, subdivided to 1, numbered by 5, as well as a shadow-square scale 0 to 12 to 0, divided to 1, numbered by 1. These scales are repeated on the other side of the subsidiary arms.