The classification of the signs in groups of three based on their elemental distinction. The signs as "fire", "water", "air" and "earth". An essential dignity in horary Astrology. Traditionally triplicities are assigned Day and Night rulers. Ptolemy devotes an entire chapter to it in Tetrabiblos (I.18, Peri Trigonon). Citing a Pythagorian rationale, that "the triangular or equilaterial form is most harmonious with itself," he discusses the "familiarity" ("sunoikiosis," friendly binding together) of the triangles. It is important to understand that Ptolemy has already explained the rationale for the positions of signs around the wheel, and that it is by virtue of these placements alone and the primacy of the triangular shape that certain signs end of in a triplicity. For example, the other two components of the Sun's triplicity would be those two signs four signs either side of Leo, which Ptolemy has already explained the Sun rules, namely, Jupiter of Sagittarius and Mars of Aries. Jupiter received Sagittarius because it has the fourth furthest orbit outside the Sun in Ptolemy's geocentric system, and so is the fourth planet placed from the Sun around the zodiac. this coincides with Sagittarius. Mars is placed in Aries because it is the third furthest planet from the Sun and so the third placed from the Moon, which is next to the Sun and marks the beginning of the feminine or nocturnal half of the zodiac. (Mars also takes the position third from the Sun in the opposite direction, Scorpio, and Jupiter also takes the position fourth from the Moon on the lunar side, Pisces.