Mercury
Mercury governs thinking, speech and the exchange of information: how a person thinks, how they explain things and how they take in what others say.
Governs
Mercury describes the way a person processes information: quickly or thoroughly, in images or in lists, out loud or silently. It also rules short trips, documents, negotiations, trade and anything to do with acting as a go-between.
Mercury never gets more than 28 degrees away from the Sun, so it is always in the same sign as the Sun or in a neighboring one. This limit is an astronomical fact, not a rule of the tradition.
In the signs and houses
By sign, Mercury shows the thinking style: in Virgo, analytical and nitpicky; in Sagittarius, generalizing; in Pisces, associative.
By house, it shows the area where a person talks, learns and exchanges the most: in the third, with their close circle; in the ninth, with the wider world; in the sixth, at work.
Strong and weak placements
Domicile: Gemini and Virgo; exaltation: Virgo. Detriment: Sagittarius and Pisces; fall: Pisces.
Mercury retrograde is a topic of its own: a period when the planet appears to move backward across the sky, three or four times a year for three weeks at a time. Tradition advises against signing contracts or buying electronics during this time. There is no evidence for this: studies find no link between retrograde periods and accidents, communication failures or failed deals.