The arrangement of signs into four groups that are
united by the same qualities and the shape of the triangle; equivalent to the elements:
fire, earth, air and water. Signs of the same triplicity are harmonious to each
other. Early sources gave each triplicity three planetary rulers: one that took
priority by day, another that governed by night, and a third that was common to
both sects– these were also used to judge the beginning, middle and end of
things. Over time the third ruler became dropped from common employment and
most texts refer to them having one planetary ruler by day and another by night