Relationship astrology is built on two main techniques: synastry and composite. Both analyse the bond between two people, but they answer different questions. Social media often confuses them. This article answers when to use which.
What Synastry Is and Says
Synastry is the technique of placing two birth charts side by side and reading the cross aspects between them. How A's Sun aspects B's Moon, where A's Mars falls in B's chart, and so on, are answered by synastry.
The core question synastry answers is: "How do two individuals react to each other?" Is the bond natural, is there chemistry, in which areas do you flow and in which do you strain.
Synastry is strongest in the early and dynamic part of a relationship. Are these two meeting well, what is the first chemistry, who triggers what in whom, all are read through synastry. Patterns like the 7th house overlay and the 8th house overlay are evaluated within synastry.
What the Composite Chart Is and Says
The composite chart is the technique of generating a single "relationship chart" by taking the mathematical midpoint of the two charts. The midpoint of A's and B's Sun is the composite Sun; the midpoint of their Moons is the composite Moon. This unified chart is read like "the relationship's own birth chart."
The core question the composite answers is: "What is the relationship itself about?" When two people merge, what personality emerges; what is the natural direction, purpose and crisis of this togetherness.
The composite tells you where the relationship goes after it forms. Long-term partnerships, marriages, business partnerships read more deeply through the composite. Synastry says "what two people feel side by side"; the composite says "what the relationship is and where it goes."
The Two Techniques Are Not the Same
Mixing synastry and composite leads to error. An example.
Two people meet. The synastry shows intense Venus-Mars aspects, high chemistry. It often gives a "soulmate" feeling. But when the same couple's composite chart is examined, the composite Sun is in a weak placement and the composite Saturn is heavy. The synastry shows "fiery chemistry," while the composite says "the relationship itself carries structural difficulty."
Reading by synastry alone, this couple looks like "definitely soulmates." Reading by the composite, "long-term difficulty." When read together, the real picture appears: there is chemistry, but the long run requires structural work.
Which One When
A practical guide.
For two people who have just met, synastry comes first. Is there chemistry, in which theme, which aspect runs hot? When the question is "do we have chemistry," synastry speaks.
For understanding what to do once a relationship is formed, the composite comes first. Where is this togetherness going, which theme defines us, where is the Sun in our composite chart? When the question is "where are we and where are we going," the composite speaks.
Family, business partnerships, long friendships, all enduring bonds, also need the composite. Synastry tells you "how two people react to each other," but where those reactions form the "object" of the relationship is the composite.
A Third Technique: Davison Chart
Its full name is "Davison Relationship Chart." Less known than synastry and composite, it is a third option. Davison finds the actual mathematical midpoint between two people's birth date-time-place and casts a real birth chart for that midpoint moment.
Davison resembles the composite but differs. The composite is a "conceptual midpoint"; Davison is a "real time midpoint" chart. Davison answers the question, "if these two people merged, at what moment would they have been born."
In practice Davison is read as a complement to the composite, not a replacement.
Reading the Three Together
A full relationship reading looks at three layers together.
First, knowing the two charts separately (each individual's natal chart). Then synastry (the cross aspects between charts). Then the composite (mathematical midpoint chart). Optionally, Davison (temporal midpoint chart) on top.
In practice most astrologers stay with the first two layers; the third and fourth are opened for deeper analysis.
What Each Technique Misses
Synastry's limit: cross aspects show the dynamic but do not directly answer "where is the relationship going." Two people may trigger reactions in each other, but whether those reactions sustain over time, synastry cannot tell.
The composite's limit: it gives the unified chart but loses the question "who feels what." Because the two individuals are not visible separately in the composite, "how do A's feelings reflect to B" is not answered there; it is still a synastry question.
Understanding the two techniques as complementary is the basic intelligence of relationship astrology. Our soulmate article opens this through applied examples.
A Practical Conclusion
Two people meeting: start with synastry. Two people in relationship: read synastry + composite together. Two people years together: composite + Davison can be added.
The Sun tells you who you are, the Moon how you heal. Synastry and composite tell you where the relationship is going. If you know your birth time and your partner's, you can look at both charts with our free birth chart tool.