A composite chart is a third chart built from two. Take each planet in your chart, take the same planet in someone else's, find the midpoint between them, and plot the result. Do that for all of them and you get a chart that belongs to neither person — it belongs to the relationship.
That is the difference from synastry in one line: synastry describes how two people meet, composite describes what the two of them become. A pair can have easy synastry and a difficult composite (they get along, but the thing they build together drags), or the reverse (constant friction between them, and a partnership that works remarkably well from the outside).
How it is built
The composite midpoint method — the common one, and the one Duetto uses — averages each planetary pair. Composite Sun sits halfway between your Sun and theirs; composite Moon halfway between the two Moons, and so on. The aspects of the composite chart are then computed among those midpoints.
Composite houses require both birth times, because they derive from a composite Ascendant built out of both charts' angles. Without times you still get a valid composite chart of planets and aspects; you do not get houses, and Duetto says so rather than filling in a guess.
What to read in it
Composite Sun — what the relationship is for. Its sign and house describe the purpose the pair keeps returning to.
Composite Moon — the emotional register of the relationship: what makes it feel safe, and what it needs in order to settle.
Composite Venus and Mars — how affection and drive express jointly: how the pair enjoys itself, and how it fights.
Composite Saturn — where the relationship carries weight, obligation, and endurance. Prominent composite Saturn is a marker of relationships that last and cost something.
The tightest composite aspects — the recurring dynamic. A composite square between Sun and Saturn describes a couple that keeps meeting the same structural question, whatever the individuals do.
An honest caveat
The composite chart is a construction, not an observation: no sky ever looked like this. Nobody was born at those midpoints. Astrologers use it because the reading it produces is often strikingly apt, which is a statement about symbolic usefulness, not about astronomy. Read it as a lens on the relationship, and drop it when it does not fit.
In Duetto
Enter both people's birth details once and you get both readings: the synastry comparison and the composite chart, computed from the same Swiss Ephemeris positions as your natal chart. Composite readings are part of the paid tier, bought with Telegram Stars in the Mini App or as an App Store purchase in the iOS app; the price is shown before you confirm.
You are entering another person's birth data — ask them first. You can delete a partner and their data at any time; see the Privacy Policy.
Duetto is for entertainment and reflection, not relationship advice.