In brief: The ruler of the sign occupied by a planet is that planet’s dispositor under the selected rulership convention.
The first link
Each sign has a traditional planetary ruler. If Venus is in Cancer, the Moon rules Cancer and disposes Venus. If Mars is in Capricorn, Saturn disposes Mars. This link follows sign location; it is not an aspect between the two planets. A calculator can draw the relationship as an arrow, but the arrow expresses a rule of chart interpretation, not energy or motion transferred between physical bodies.
Follow the chain
To continue, find the sign containing the dispositor and identify that sign’s ruler. Venus in Cancer points to the Moon; if the Moon is in Virgo, it points to Mercury; if Mercury is in Gemini, Mercury rules its own sign and the chain ends there. This terminal planet is often called a final dispositor. Not every chart has one: a closed loop or several separate chains can prevent a unique terminal ruler.
State the ruler system
Traditional rulers assign Mars to Scorpio, Jupiter to Pisces, and Saturn to Aquarius. Some modern readers add Pluto, Neptune, and Uranus to those signs. Changing that mapping can change the chain or its endpoint. A reproducible dispositor diagram therefore labels its rulership table and says whether outer-planet co-rulers were included. It should not quietly combine the schemes while claiming one definitive final dispositor.
Use in interpretation
The chain can direct attention to another planet’s sign, house, and aspects when reading a placement. It is a way to organize chart questions, not a numeric score of personality or fate. If a birth time is unknown, sign-based links may remain calculable while houses and the Ascendant do not. If a fast body is near a sign ingress, check the time range before accepting the next arrow in the chain.