Decan

Each 30° sign divides into three 10° decans, each said to tint the sign with a secondary 'sub-ruler'. A way the same sign carries three slightly different flavors.

A decan (or decanate) is one of the three 10° slices of a sign: 0°–10°, 10°–20°, 20°–30°. A planet keeps its sign but picks up the tone of the decan's sub-ruler, so two people with, say, the Sun in Cancer can read a little differently depending on the degree. There are two main traditions, and they disagree about who rules each decan. The modern 'triplicity' system assigns the rulers of the three same-element signs in order (so Cancer's decans are Moon, Mars, Jupiter). The older Egyptian/Chaldean 'faces' system runs the seven classical planets in Chaldean order around the whole zodiac (so Cancer's faces are Venus, Mercury, Moon). We show both and privilege neither. Because your reading already computes the exact degree of each placement, it can show your decan in both systems — a concrete example of how a single sign is never just one thing.

Not: A decan is not a more 'precise' or scientific reading of a sign. It's a finer symbolic subdivision — and since two traditions assign it differently, it's plainly a lens, not a measurement.

Like all sign symbolism, decans are inherited convention with no demonstrated mechanism. The two systems disagreeing for the same degree is a useful reminder of that.