To start from the beginning of an explanation, please read that article: Motion of the Planets
Mercury rules Gemini and Virgo; it is exalted in Virgo, has its fall in Pisces and in detriment in Sagittarius.
Venus rules Taurus and Libra; is exalted in Pisces, in her fall in Virgo and in detriment in Aries and Scorpio.
Mars rules Aries and Scorpio; is exalted in Capricorn, has his fall in Cancer and in detriment in Taurus and Libra.
Jupiter rules Sagittarius (* and Pisces); is exalted in Cancer, has his fall in Capricorn and in detriment in Gemini.
Saturn rules Capricorn; is exalted in Libra, has his fall in Aries and in detriment in Cancer.
Uranus rules Aquarius; is exalted in Scorpio, has his fall in Taurus and in detriment in Leo.
Neptune rules Pisces (** and Sagittarius); is exalted in Gemini, has his fall in Sagittarius and in detriment in Virgo.detr
A planet is strong and has a beneficial influence when in its own house or when exalted, but is weak and non-beneficial when in its fall or detriment and very weak when retrograde.
A pIanet is said to be strong or to have essential dignity when in its own triplicity. Thus: Mars and Jupiter are fiery planets; Aries, Leo and Sagittarius are fiery signs; therefore Mars, Jupiter and the Sun have more power in these signs than in others. Saturn is Earthy and is strong in Taurus, Virgo, and Capricorn.
A planet has strong signification and exerts a more marked influence when located in the cardinal signs of the figure, as these signs form the principal angles of the horoscope.
The strength is considerably mitigated when located in common (*** mutable) signs-Gemini, Virgo, Sagittarius, and Pisces – as when placed in such signs their action in bringing about events is sluggish and at times fails entirely, while if located in Cardinal signs – Aries, Cancer, Libra, and Capricorn they bring about a more prompt and decided result, while if located in Fixed signs–Taurus, Leo, Scorpio, and Aquarius – their influence is much more slow in culminating but more lasting when completed.
Planets located in the northern signs – from Aries to Libra-exert a stronger influence in bringing about events then when located in the other portion of the Zodiac, for the reason that when in northern signs they aro in north declination, therefore exert a more direct influence upon the northern hemisphere; while in southern signs their declination is south and their influence exerted more directly upon the southern hemisphere.
From Guide to Astrology by Fredrick White
The Table of Essential Dignities
* added by Alexander Demin
** added by Alexander Demin
*** added by Alexander Demin
The next related article: The Houses of the Horoscope