The planet that rules the sign that another is currently in is called its planetary dispositor. The conditions of the dispositor can influence any planet it disposes.

Placeholder for Dispositor Tree.

This is a planet that, directly or indirectly, disposits all other planets in the chart, and is itself is in its own sign (disposits itself). It has enormous influence over the rest of the chart.

Mutual Reception is when two planets are in a sign that the other rules. For example, Venus in Scorpio and Mars in Taurus.

Each sign is said to be "ruled" by a planet. The ruling planet is most able to express it's purest energy in the sign(s) that it rules.