Mercury is how you think and how you speak. Not what you think, that is the rest of the chart, your history and your choices. Mercury is the format: the speed, the way of organising, the manner of saying, and also of understanding what is said to you.

And here is what almost no text explains, though it is the most useful fact about this piece: your Mercury can only be in three signs, your Sun's, the one before or the one after. Mercury never moves more than 28 degrees away from the Sun, and a sign has 30. There is no Sun in Aries with Mercury in Leo, and no calculator will say otherwise.

That is what answers the question that comes up all the time: why two people of the same Sun sign think in such different ways. Sun in Aries with Mercury in Aries decides on the spot and says what it decided; Sun in Aries with Mercury in Pisces decides on the spot and cannot explain why; Sun in Aries with Mercury in Taurus looks impatient and takes three days to answer an email.

Having Mercury in Gemini means you think fast and in many directions at once. This is the position in which Mercury is at home: Gemini is its domicile, and here everything Mercury does is done at full volume.

Mercury is the planet of thought and speech; in Gemini, it becomes light and mobile. This person learns anything in three weeks, changes subject with no warning, and links points nobody had linked, because they always have ten windows open.

And here is what hardly any text writes down: they are not distracted: they are processing in parallel. What looks like scattering is usually the only way this head has of working. Forcing it to one subject does not make it more productive: it switches it off. And that is why so many people with this position think they are stupid at school and brilliant after it.

There is a second piece that changes everything: which house this Mercury fell in, the area of life in which this head works. Mercury in Gemini in the 3rd house doubles the subject: somebody made of language. In the 8th, somebody who investigates and connects what nobody wanted linked. The sign says HOW; the house says WHERE.

It shows in how many subjects this person keeps open at the same time. These are concrete signs, the kind you recognise in your own week:

• learns fast and on their own, and collects useful half-knowledges
• starts talking about one subject and finishes on another, without noticing
• needs varied stimulation, and boredom knocks them down more than tiredness
• writes or talks in order to understand what they think
• reads three books at once and finishes all three
• copies accent, manner and vocabulary without meaning to

Notice what this list does not say. It does not say the person is shallow nor scattered, two labels this position carries and which describe the excess, not the position. It says their thinking works in parallel.

The strength is joining the dots: near this person things connect. They see the likeness between two fields that do not speak to each other and translate one into the other, and that is the raw material of almost every new idea.

In practice that turns into three useful things: a capacity to learn that makes them useful in any field within three weeks; an ease in explaining the difficult thing, because they had to understand it on their own; and a sociability that opens doors, since they talk to anybody about anything.

That strength does best in communication, writing, teaching, translation, sales, journalism, technology and any work where varying is part of the job. And it does badly in a repetitive long-term task, in a mute environment and in any place where changing subject is treated as a lack of commitment.

There are two, and they are opposites: the excess is variety turning into scattering, and the deficiency is whoever was convinced they think wrongly and started imitating another Mercury. The Shelly Method reads every archetype through four faces, the light, the shadow, the excess and the deficiency, and it is the last of them that hardly any text mentions.

The excess is starting everything and finishing nothing: five courses open, three books half read, a head that will not switch off at night. It also shows up as a dangerous facility with words, the person argues too well and ends up convincing themselves of things they do not believe.

The deficiency is the opposite and hurts more, and here it has a recognisable origin: school rewards two Mercuries and punishes the other ten. It rewards Virgo and Capricorn: method and focus. A Mercury in Gemini that took a beating for being scattered learns to force itself onto one subject, to mistrust its own curiosity and to call immaturity what was the engine, and becomes somebody bored and unproductive, who is neither focused nor free.

Both faces have the same raw material, and that is why the repair is the same: a variety that has to finish something, and not be pruned. It is not solved by asking the person to concentrate on one thing only, they switch off, and the attempt produces the deficiency described above. It is solved with three projects instead of ten, and one of them reaching the end, and that is what Mercury's house and the rest of the chart show.

It matures when the person discovers that finishing one thing does not close the others, and this discovery is worth more than any attempt to learn focus. Pruning curiosity by effort almost always fails, and produces the deficiency described above.

There is a rhythm that helps understand the arc, and it belongs to Mercury itself: three or four times a year it goes retrograde, for about three weeks. It is not a punishment: it is the period in which thought naturally goes back, revises, rereads, finds again. For whoever has Mercury in Gemini, those weeks stir things more than average, because the planet rules the sign: misunderstandings, a lost text, a conversation that has to be had again. And if you were born with Mercury retrograde, and about eighteen per cent of people were, that is your way all year round: saying, hearing it back and revising until you get where you were going.

Maturity here is not learning to have one subject only. It is the person coming to leave a trail. A mature Mercury in Gemini keeps ten windows open, and starts closing one at a time with something finished inside, and it is that trail that turns scattering into work.

No honest text gives an age for this. The arc depends on the whole chart and on the life the person had.

It says how you think, and not what you think nor how much you know, and that distinction is the one popular astrology makes worst. Mercury is the format; the content is your life.

The house this Mercury fell in changes the area in which this head works: Mercury in Gemini in the 9th house is somebody who studies and teaches for a lifetime; in the 6th, somebody who handles the everyday with an agility nobody can keep up with. See the series The Sun in the 12 houses to understand what each house means.

Your Sun's sign is next to this one or the same, and the combination of the two is what produces the difference you feel in relation to other people of your sign. See the series The Sun in the 12 signs.

And there is the Moon, which says what you need inside. Mercury in Gemini with a Moon asking for the opposite produces somebody who thinks one way and feels another, and it is that mismatch, and not the sign on its own, that usually explains the complaint. See the series The Moon in the 12 signs.

A limit said out loud, because it is also part of reading seriously: none of this is a diagnosis. Astrology describes tendency and gives vocabulary for what the person already lives; it does not measure intelligence, does not identify a learning difficulty and does not replace a professional assessment.

And the questions that come up most on this subject, with the short answer: