What Mercury governs
Mercury describes how someone takes in information, sorts it and passes it on. That covers speech and writing, but also commerce, short journeys, documents, siblings and the practical business of getting a message from one place to another. It says nothing about intelligence. It describes a method, and methods that look slow in one context are precise in another.
Read alongside the Moon it shows whether a person reasons through a problem or reacts to it first. Read alongside the third and sixth houses it shows how that method is applied to daily work.
Tied to the Sun
Mercury orbits inside the Earth's orbit, so from here it never appears more than about twenty-eight degrees from the Sun. In chart terms it can only occupy the solar sign, the one before it or the one after it. A person with the Sun in Virgo can have Mercury in Leo, Virgo or Libra, and nothing else.
This also rules out most aspects. Mercury can never oppose or square the Sun, and the only major aspect available between them is the conjunction. A Mercury conjunct the Sun within a few degrees is called combust in traditional practice, which is read as the thinking being absorbed into the solar purpose rather than standing apart from it.
Speed, and how personal the sign is
Mercury spends roughly two to three weeks in a sign when moving directly, though a retrograde period can stretch that to two months or more. The sign is therefore personal in the ordinary sense, shared with people born within a few weeks, and it is not generational.
Because it can outpace or lag the Sun, Mercury is one of the placements most often guessed wrongly from a birth date alone.
Rulership, dignity and retrograde
Mercury rules Gemini and Virgo, and is exalted in Virgo as well, which is unusual. It is in detriment in Sagittarius and Pisces and in fall in Pisces. Retrograde motion happens three or four times a year for about three weeks at a time, so somewhere around one person in five is born with it.
That share matters. A natal Mercury retrograde is far too common to be treated as a defect. It is read as a mind that revisits and reworks material rather than settling it on first pass, and it often comes with a preference for writing over speaking.
The house, which needs a time
The house tells you what the perception is spent on. Mercury in the second house thinks about resources and value; in the eleventh it thinks in groups and networks. The sign gives the style, the house gives the subject, and only a birth time produces the house.
Where Mercury falls in the same house as the Sun, which is common given the distance limit, the two readings tend to merge and the aspect matters more than the house.
Questions people ask about this
Can Mercury be in any sign?
No. It stays within about twenty-eight degrees of the Sun, so it falls in the solar sign or one adjacent to it.
How common is a natal Mercury retrograde?
Mercury is retrograde three or four times a year for about three weeks, which works out at roughly one birth in five.
Which signs does Mercury rule?
Gemini and Virgo. It is also exalted in Virgo, and in detriment in Sagittarius and Pisces.
Does Mercury show how clever someone is?
No. It describes a method of taking in and passing on information, not a level of ability.