I have mentioned elsewhere which planet rules which sign. I like to expand on that now, adding the opposite end.
Rulership and Exaltation are where/when the planets are dignified. They feel at ease, perform well, at their maximum strength. They act as ‘hosts’ more than ‘guests’. They are called ‘Essential dignities’.
Detriment and Fall are where/when the planets are debilitated. They do not perform well, without their willpower or strength. They are called ‘Essential debilities’.
Following is the table for an overview. (Day/Night assignment for rulership and exact degrees of exaltation are omitted here).
Rulership Exaltation Detriment Fall
Aries Mars Sun Venus Saturn
Taurus Venus Moon Mars
Gemini Mercury Jupiter
Cancer Moon Jupiter Saturn Mars
Leo Sun Saturn
Virgo Mercury Mercury Jupiter Venus
Libra Venus Saturn Mars Sun
Scorpio Mars Venus Moon
Sagittarius Jupiter Mercury
Capricorn Saturn Mars Moon Jupiter
Aquarius Saturn Sun
Pisces Jupiter Venus Mercury Mercury
There is only one position (or degrees) for exaltation, but two signs for rulership, except for the Sun ruling just Leo and the Moon ruling just Cancer.
To determine the debilities, let’s look at detriment first. The rule is:
one sign’s ruler is its opposite sign’s detriment.
For example, Aries’ ruler Mars must be Libra’s detriment, as they are 6 signs away = 180 degrees opposition one another. Similarly, Scorprio’s ruler Mars must be Taurus’ detriment as they are 6 signs away.
In the other direction of the above pairs: Libra’s ruler Venus is Aries’ detriment. Taurus’ ruler Venus is Scorpio’s detriment. In this fashion, you can fill all the detriments above.
As for the assignment of fall:
one sign’s exaltation is its opposite sign’s fall.
There are only 7 falls as there are only 7 exaltations.
Remember: rulership is more dignifying than exaltation. Thus, detriment is more debilitating than fall.
In conclusion: knowing the dignities/debilities, you can know within your natal chart which planets are better disposed to perform a task, so to speak. Also you can rate the current sky in the same manner. Currently Mercury is in Virgo and Venus is in Libra, i.e. both are in their own rulership.