In astrology, a planet's domicile (or less commonly house, not to be confused with the astrological house system) is the zodiac sign over which it has rulership. It is important to note that this is a separate concept from the houses of the horoscope. A planetary ruler is given to each sign, over which the planet is said to have a more powerful influence when positioned therein. A planet is considered to be in domal dignity when it is positioned in the sign it rules. This is the strongest of the five essential dignities of a planet.
Conversely, Saturn, the most distant (and hence the "coldest") of the planets known to ancients, was accorded the rulership of Aquarius and Capricorn, the signs opposite Leo and Cancer, respectively.
Jupiter, being next furthest away, was given the signs on either side of Aquarius and Capricorn (Pisces and Sagittarius), and Mars, next in order, received the next two (Aries and Scorpio).
Since Mercury never appears more than one sign from the sun in either direction, it was deemed to rule the two signs on either side of Leo and Cancer (Virgo and Gemini), and since Venus can never be found more than two signs from the Sun, it obtained the rulership of Libra and Taurus.
The discovery of planets outside of the geocentric field of vision in modern times provided a dilemma for astrologers, which most eventually resolved by a general consensus declaring Uranus to be Aquarius's ruling planet, while assigning Neptune to Pisces and later, Pluto was given to Scorpio.
Some modern authorities uses the concept of "night rulerships" to find room for the additional dignities. Uranus was designated the day ruler of Aquarius while Saturn was to be its night ruler. Similarly, Neptune was the day ruler of Pisces, leaving Jupiter as the night ruler, and Pluto was the day ruler of Scorpio with Mars as the night ruler. This notion probably had its origin in the concept of astrological sect, but this was the only vestige of sect in the modern tradition. Unfortunately, no allowance was made for these modern suggestions in the complex traditional doctrine of essential dignities, but the idea was very popular.
The use of dual rulerships in a manner such as this was also be known as "co-rulership". Some astrologers believed that the new co-rulers were primary rulers of the signs with which they were associated and may have been sole rulers of those signs, and if that was the case, two other planets, one linked to Libra or Taurus, and the other to Virgo or Gemini, may await discovery, thus eliminating the need for dual rulership of a sign altogether. Although the status of Ceres is unknown at the moment in astrology, due to its new definition as a dwarf planet, it has been suggest as the ruler of Virgo or Taurus. For some modern astrologers it is the ruling planet of Virgo and co-ruler of the 6th house with Mercury, and for some others the ruling planet of Taurus and the 2nd house with Venus.The possibility exists that it isn't involved with any sign, but in any event, it can almost definitely be attributed to the Earth element.
When a person attempts to determine the rulership of a house, the more planets or bodies that rule that house will reveal its complexity. Generally, the sign on the cusp will be the outward representation of the rulership, but the ruler of the sign inside the house will not be readily apparent, but will manifest itself at critical points in a person's life.
As in the case of intercepted houses, the sign that is in the middle of the house is the sign that the native must transcend before he can progress to the next phase of the house's rulership. Therefore, as in the previous example, if a house is intercepted with the sign of Scorpio then the native must will be initially presented with many Libran qualities, but must work through those very intense issues of sex, death and regeneration before he will be able to enjoy the benefits of Jupiter, which will be the last and ultimate state the native must work through before he has fully experienced all of the aspects of that house.
| Sign Domicile | Ruling planet (ancient) | Ruling planet (modern) | Sign Detriment |
|---|---|---|---|
| Aries | Mars | Libra | |
| Taurus | Venus | Ceres | Scorpio |
| Gemini | Mercury | Sagittarius | |
| Cancer | Moon | Capricorn | |
| Leo | Sun | Aquarius | |
| Virgo | Mercury | Pallas/Chiron/ Ceres/Vesta | Pisces |
| Libra | Venus | Aries | |
| Scorpio | Mars | Pluto | Taurus |
| Sagittarius | Jupiter | Gemini | |
| Capricorn | Saturn | Cancer | |
| Aquarius | Saturn | Uranus | Leo |
| Pisces | Jupiter | Neptune | Virgo |
A planet's detriment is the sign opposite to its rulership or domicile, considered a negative aspect. However, a detriment is not as averse as its fall. 
Furthermore, the ancients assigned domal dignity to the North and South nodes of the Moon, although these do not qualify as rulerships since the nodes do not have governance over any signs. However, the nodes are said to act more comfortably and reliably in those places:
Some discontent exist within the Western astrological community about some of the signs that are often still said to share rulers. Many suggestions have been made, but consensus could actually take centuries, as it did in the cases of Uranus and Neptune.
On the other hand, most modern, psychologically-oriented astrologers do use the outer planets as ruling planets of those particular three signs listed above. Many others also use the ancient rulers while using the modern planets as the co-ruler of those signs.
Other astrologers do not use any of the planets as sign rulers(Cosmobiologists, Uranian astrologers, Hamburg School of Astrology, other groups/individuals), preferring to focus more on the astrological aspects and other portions of the horoscope instead.
Most astrologers practicing Vedic astrology (Jyotish) only use the traditional rulership system and have been doing so consistently for quite some time.