Synastry is the reading of two charts crossed: where one person's planet falls in the other's chart, and what angles the two form between them. The word comes from the Greek synastria, “stars together”, and the technique is as old as the individual chart.

What makes it another thing is the crossing. One chart describes one person; two charts describe two people. Synastry is not the sum of the two, it is a third object, one that exists only when the two meet and that describes the meeting, not each of them separately.

That is why the same person looks different in two synastries. It is not pretence: it is that her Mars falls in one person's house 3 and another's house 8, and the same force arrives as conversation in one case and as a grave matter in the other.

Synastry looks at three layers, and they have a hierarchy. Whoever reads out of order gets lost in the volume, and the volume is large.

• The contacts between planets. One person's Sun with the other's Moon, one person's Venus with the other's Mars, and so on. They are angles: conjunction, opposition, square, trine and sextile.
• The house overlay. Which area of one person's life the other's planets fall into. It is the layer that “sign with sign” does not have, and usually the one that explains what the person feels.
• The exact distance of each contact. An aspect of half a degree is another thing from one of five and a half. This is the layer almost every text on the internet ignores, and it is the one that separates a reading from a listing.

The contacts that weigh most are those of the luminaries and the personal planets, Sun, Moon, Mercury, Venus and Mars. Saturn, Uranus, Neptune and Pluto move slowly and stay in nearly the same place for years: the Neptune of someone born in 1990 makes the same aspect to your Sun as the Neptune of everyone born in that stretch. A contact between slow planets describes a generation, not a couple.

Counting aspects says nothing, because two charts drawn at random already have 26.7 aspects between them on average, and never have zero. I measured this across 200 thousand pairs of dates between 1966 and 2025, crossing ten planets of one with ten of the other, at the six-degree orb commonly used.

Between any two charts, the average is 26.7 aspects, with a 6° orb; out of 200 thousand, the pair with the fewest aspects had 3 and the one with the most had 56, and no pair without an aspect turned up; 46.4% of pairs have a Sun–Moon contact, 46.2% have a Venus–Mars contact and 71.2% have at least one of the two.

Look at the last number. Seven in ten people you pass on the street have, with you, a Sun–Moon or a Venus–Mars contact, the two the internet calls a sign of soulmates. If we widen the orb to eight degrees, the average number of aspects rises to 35.6.

That does not disprove synastry. It disproves one way of using it: the counting one. “We have eleven aspects!” is a sentence with no content, because eleven is below the average. What sets one meeting apart from another is not the quantity, it is which contacts exist, between which planets, at what distance and falling in which house.

It is also why an app that returns “87% compatibility” is not measuring anything defensible. It added up points on a list that was nearly full before the two of you met.

What a reading does with the 26.7 is to weigh, not add up. Of the hundred possible pairs, most are of no interest: one person's Pluto with the other's Neptune describes the decade both were born in. What is left after that cut is a handful of contacts, usually between three and eight, and those are what gets written about. A well-read synastry is shorter than the list of aspects, not longer.

The overlay of houses shows which area of your life the other person comes into, and it is the part of synastry most recognisable from experience. The other person's planets fall into houses of yours, and the house is the subject.

Three examples, with the same Venus landing in different places:

• Her Venus in your house 5. The relationship looks like dating even when it is not: fun, creation, the urge to show the person off. It is the overlay that most resembles passion.
• Her Venus in your house 10. The person shows up attached to your public life, your career or your reputation. Plenty of people discover that they fall for whoever they admire professionally by reading exactly this line.
• Her Venus in your house 12. The same Venus, and now the relationship has something private about it, either because it happens away from other people, or because it stirs what you do not look at head-on.

And the overlay is asymmetric, which is the most useful discovery of this technique. Her planets land in his houses, and his in hers, and the two lists are different. A relationship where he comes into her house 7 and she comes into his house 12 is one where the two are living different things, and that is the conversation synastry makes possible.

Without the birth time of both there are no houses, and that whole layer disappears. It is the reason synastry demands the hour, and not only the date.

Synastry is not only for couples, and restricting it to couples is wasting the technique. Crossing two charts reads any relationship between two people: friendship, partnership, mother and daughter, boss and employee. What changes is not the method, it is what you look for inside it.

In a couple, what weighs is Sun with Moon, Venus with Mars and houses 5, 7 and 8; in friendship, Moon with Moon, Mercury and house 11; in a business partnership, the partner's house 7, the business's house 10 and the shared money's house 8; and between mother and daughter, the Moon, house 4 and one's Saturn in the other's house 10.

Notice that friendship asks for other pieces, and not for the same pieces in a weaker form. Among friends Moon with Moon is usually worth more than Venus with Mars: what holds a twenty-year friendship together is being at ease in the other's presence, not desire. And Mercury is what tells you whether the two of you talk or merely like each other.

In a partnership the question changes again, and gets more concrete: who takes the risk, who looks after the shared money, and what happens when the business gets tight. That is read in houses 2, 8 and 10 crossed, and not in liking each other.

Synastry does not say whether it will work out, and whoever promises that is selling something else. Synastry describes the material the two work with, not what they will build out of it, which is a decision two people make over years.

Four limits, said out loud:

• A hard aspect is not a sentence. One person's Saturn over the other's Venus describes a relationship that demands and that matures slowly. Many long marriages have that contact, and many short passions have none at all.
• An easy aspect is no guarantee. Trines describe what runs by itself, and what runs by itself is also what nobody tends.
• Each person's own chart still holds. Whoever has Venus in fall in their own chart takes that into every relationship, and no synastry solves from outside what belongs inside.
• Astrology does not decide. It does not tell you to stay or to leave, diagnoses nobody and does not replace couples therapy. It gives vocabulary for a conversation that was already hard to have.

And there is the limit this blog repeats in every text, which here takes a shape of its own: two people with the same synastry do not live the same relationship. Age, history, what each has already lived and the moment they met change everything. The sky describes the configuration; life happens inside it.

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