Astrological rule when there is no planet in the sign of a planet.

 
Vacant houses in astrology.

A planet, in whose sign there is no planet, would give its own independent results, according to its own nature or relation with the Ascendant, but when any of the signs are occupied by a planet, particularly, by a natural malefic planet, then it would give the results of the planet occupying its sign. This is a most important rule which is of universal use.
For example, Mars in located in Sagittarius, then Jupiter,lord of Sagittarius, would give very little independent results, as a natural benefic planet. It would act as Mars and would, therefore, harm the life prospects of the house etc., it may aspect. For example, it may be aspecting the 8th house. In that event, as a single factor,Jupiter’s aspect on that house be considered as the Cruel aspect of Mars, denoting death by injury, violence,Fire etc., and not by ordinary circumstances as would Have been the case otherwise.Similarly, suppose in the case of an Aries nativity,Mars and Sun occupy the sign Pisces in the l2th house and Jupiter aspects the 8th house, then even though Jupiter is lord of a watery house ( 12th) and a watery sign (Pisces) contained therein, it will act as fire and cause death by fire. The reason is that Jupiter here acts as fire by virtue of its Lordship over a sign that contains fiery planets Mars and Sun.