This is a planet that, directly or indirectly, disposits (or some say "disposes") all other planets in the chart. It is always in the zodiac sign that it rules (disposits itself). A planet disposits another planet when the latter planet is located in a sign that the initial planet rules. For example, any planet in the sign of Gemini has Mercury as its dispositor.
It's possible to draw a Dispositor Tree that shows each planet linked to its dispositor (the ruler of the sign it's in), and in turn its dispositor is shown linked to its dispositor, and so on. If all planetary dispositors link back to only one planet which is in its own sign of rulership, then that planet is called the Final Dispositor.
In order for a chart to have a Final Dispositor, there must be:
- only one planet in its own sign,
- at least one more planet in that sign or in another sign ruled by that planet
- no Mutual Receptions in the chart
- all planets must trace their rulership back to that one planet in its own sign
- only one rulership system used – traditional or modern – you can't "mix and match."
By definition, there cannot be more than one Final Dispositor in the chart. If two other planets in the chart are in Mutual Reception (in each other's sign) then even if all other planets trace back to just one planet, there is no single Final Dispositor.
The Dispositor Tree diagram in the top-right shows Mercury as the Final Dispositor, and uses traditional rulerships. Note that if this tree were to use modern rulerships, Mercury would not be the Final Dispositor, since Uranus in Pisces and Neptune in Aquarius would be considered a Mutual Reception (in each other's sign of rulership).
The second Dispositor Tree at right uses modern rulerships and shows the Sun as the Final Dispositor. Note that in this case, the Sun is the Final Dispositor even if traditional rulerships were to be used. In that case, Neptune in Aquarius would be disposed by Saturn instead of Uranus.
Most charts do not have a Final Dispositor, so when this does happen, that planet has a lot of influence over the rest of the chart. It's like "all roads lead to Rome" in the chain of dispositors. The Final Dispositor has the "final say" in the transfer of energy from one planet to another through rulership.