Synastry explained: how two birth charts are compared
The technique is more interesting than the compatibility score it usually gets reduced to — and a good deal less conclusive.
6 min read · Published 16 August 2026
The short answer
Synastry is the comparison of two birth charts to describe a relationship between two people. It works in three main ways. Cross-aspects measure the angles between one person's planets and the other's, so your Venus might sit at 90 degrees to their Mars. House overlays place one person's planets into the other's houses, showing which areas of that person's life the other activates. And the composite chart, a different technique often grouped with it, produces a single third chart from the midpoints of both to describe the relationship as an entity. Traditionally the strongest signals are contacts involving the Sun, Moon, Venus, Mars and the angles, particularly the Ascendant and Descendant. What synastry cannot do is determine whether a relationship should continue, and no comparison technique has been shown to predict relationship outcomes in testing.
The three techniques, kept apart
These get blurred together constantly, and they answer different questions.
Cross-aspects are the core of synastry. You take the angle between a planet in chart A and a planet in chart B — the same aspects used within a single chart, applied across two. A trine between your Moon and their Venus, a square between your Mars and their Saturn. This describes how the two people act on each other.
House overlays drop one person's planets into the other's houses. Their Jupiter landing in your tenth house says they touch your career and public standing; your Saturn in their fourth says you touch their sense of home. Overlays are asymmetric — what they do to you is not what you do to them — and that asymmetry is often the most interesting part of a reading.
Composite charts are a different animal. They average the midpoints of both charts to produce a third chart, read as the relationship itself rather than as either person. It is a mid-twentieth-century development, not a traditional technique, and it answers a different question: not how you affect each other, but what the thing between you is like.
Which contacts the tradition weighs most
| Contact | Traditionally read as |
|---|---|
| Sun–Moon | The classic pairing; mutual recognition, ease of daily life |
| Venus–Mars | Attraction and physical chemistry, in either direction |
| Moon–Moon | Emotional rhythm; similar needs, or a persistent mismatch |
| Sun–Ascendant | Being seen; one person visible to the other from the start |
| Venus–Saturn | Commitment or coldness, depending on the aspect and much else |
| Mars–Saturn | Friction, frustration, and the most common hard-work contact |
| Moon–Saturn | Security or heaviness; often cited in long marriages and in unhappy ones |
The last row is worth dwelling on. The same contact appears in accounts of enduring marriages and of miserable ones, which tells you the technique is describing a texture rather than an outcome — and that any reading claiming to predict which one you have is going beyond what the tradition supports.
Why easy charts are not the good ones
The intuition most people bring is that lots of trines means a good match and lots of squares means a bad one. The tradition does not say that, and the reasoning parallels the hard and easy aspects within a single chart.
Contacts described as harmonious produce ease, familiarity and comfort. Contacts described as hard produce friction, charge and the sense that something is at stake. Relationships people describe as consuming tend to be heavy on the second kind, and relationships people describe as restful tend to be heavy on the first. Neither is the good one; they are different experiences, and which suits a person is not something a chart adjudicates.
This is also where synastry gets misused most often — a difficult contact read as a warning to leave, or an easy one read as a guarantee to stay. Both mistake a description for a verdict.
Where the compatibility percentage comes from
Almost every site that offers a synastry score has invented its weighting. There is no traditional formula that converts cross-aspects into a percentage, because the tradition does not work in percentages.
What a score actually encodes is a set of decisions someone made: which contacts count, how much each is worth, what orb is allowed, whether house overlays are included and how the outer planets are treated. Change any one and the number changes. Two reputable sites will give the same couple materially different scores, which is the clearest available evidence that the number is a presentation choice rather than a measurement.
AsterNym shows a match figure and treats it as a conversation starter rather than a verdict, which is the only honest thing to do with a number of that kind. A percentage is a good way to begin a conversation and a bad way to end one.
Using it without letting it decide things
Read overlays in both directions before concluding anything. The asymmetry — that you land in their fourth house while they land in your tenth — is usually more informative than any single aspect, and it often describes a real imbalance in what each person is getting.
Weight the tight contacts and ignore the loose ones, for the same reason that applies within one chart: generous orbs let a comparison say whatever was wanted.
Discount outer-planet contacts between people of similar age. If you and your partner were born four years apart, your Neptune contact is shared with everyone in that cohort and says nothing about the two of you specifically.
And keep the technique in its place. Synastry is good at giving language to a dynamic two people already sense. It is not evidence about whether someone is good for you, and where it has been tested against relationship outcomes — including large studies of marriage and divorce records against sun-sign and chart comparisons — it has not predicted them.
Common questions
- What is synastry in astrology?
- The comparison of two birth charts to describe a relationship. It measures cross-aspects, the angles between one person's planets and the other's, and house overlays, where one person's planets fall in the other's houses. The composite chart, which averages both into a third chart, is a related but separate technique.
- What is the difference between synastry and a composite chart?
- Synastry compares two charts as two charts, describing how the people affect each other. A composite averages the midpoints of both to create a single third chart, read as the relationship itself rather than as either person. Composite charts are a mid twentieth century development rather than a traditional technique.
- Which synastry aspects are the strongest?
- Traditionally those involving the Sun, Moon, Venus, Mars and the angles, particularly the Ascendant and Descendant. Sun to Moon contacts are the classic pairing, Venus to Mars is read as attraction, and Saturn contacts are read as either commitment or heaviness depending on the aspect and the rest of the comparison.
- Are compatibility percentages meaningful?
- They are presentation choices rather than measurements. No traditional formula converts chart comparison into a percentage, so every score reflects decisions about which contacts count and how much each is worth. Two reputable sites will give the same couple noticeably different numbers, which shows what the figure really is.
- Can synastry tell me if a relationship will last?
- No, and any reading that claims to is overreaching. The same contacts appear in accounts of long happy marriages and of unhappy ones, so the technique describes texture rather than outcome. Studies comparing chart matching against marriage and divorce records have not found predictive value.