In traditional astrology, each triplicity has several planetary rulers, which change with conditions of sect--that is, whether the chart is a day chart or a night chart.
Triplicity rulerships are a very important essential dignity--one of the several factors used by traditional astrologers to weigh the strength, effectiveness and integrity of each planet in a chart. Many Hellenistic astrologers (for example, Dorotheus of Sidon) considered triplicity rulership the most powerful and demonstrable of the several essential dignities of a planet.
Triplicty rulerships (using the "Dorothean system") are as follows:
|Triplicity||Day Ruler||Night Ruler||Participating Ruler|
|Fire (Aries, Leo, Sagittarius):||Sun||Jupiter||Saturn|
|Air (Gemini, Libra, Aquarius):||Saturn||Mercury||Jupiter|
|Earth (Taurus, Virgo, Capricorn):||Venus||Moon||Mars|
|Water (Cancer, Pisces, Scorpio):||Venus||Mars||Moon|
One way in which triplicty rulerships were used by earlier astrologers was to divide a person's life into three periods: early, middle, and late. The condition of the triplicity ruler that is in sect in the chart is evaluated when considering the tenor of the early part of life; the ruler out of sect is examined for the middle of life; and the last third of life is evaluated by looking at the condition of the participating triplicity ruler. "Participating" rulers were not used after the Hellenistic period.
In medieval systems of astrology, each essential dignity was given a different weight. Domicile rulers were given 5 points of weight; exaltation rulers were given 4 points; and triplicity rulers were assigned 3 points of weight. This gives some idea of how much power medieval astrologers accorded to each essential dignity.
Read more about this topic: Triplicity, Method of Organization