lodestarYou may read that Gemini is ruled by Mercury or Aries by Mars. In astrology, a ruling planet is a named link between a sign and a planet. It does not mean the planet controls a person.
A link on the chart's symbolic map
Western astrology assigns a ruling planet to each sign. This gives astrologers another reference point when reading a chart. It is a convention for interpretation, not a force that makes someone act a certain way.
Two pairs to start with
Mercury is linked with thinking and communication, and is Gemini's ruler. Mars is linked with action and initiative, and rules Aries. These pairs help explain the chart's language; they do not tell you what every Gemini or Aries person is like.
How a reader uses the link
If a chart has a planet in Gemini, an astrologer may also look at Mercury for more context. That is an extra reading step, not a verdict about the person. For a first look, remembering the sign-planet pair is enough.
A few common questions
Why do some ruler lists differ?
Traditional and modern astrology use different rulers for a few signs. For example, traditional lists give Aquarius to Saturn, while many modern lists use Uranus.
Sources & further reading
Astrology sources describe their own traditions and methods; they do not establish predictive accuracy. The writing exercises are original Lodestar editorial material.