Composite chart or synastry: which one answers your question

These get treated as two flavours of the same thing. They are not, and picking the wrong one is why a lot of relationship readings feel like they answered a question nobody asked.

Synastry: what you do to each other

Synastry lays your two charts over one another and reads the contacts. Their Venus on your Moon. Your Saturn on their Sun.

It is directional, which is the important bit. It tells you who does what to whom. The Mars person pursues, the Venus person is pursued. The Saturn person steadies or restricts, the Moon person is steadied or restricted. If your question is why does this keep happening between us, synastry is the technique that answers it.

Composite: the relationship as its own creature

A composite chart takes the midpoints between your two charts and builds a third one from them. Not you, not them, but the entity the two of you make.

It is not directional at all. Nobody in a composite is doing anything to anybody. It describes the relationship's own character: what it is for, what it tends to be about, where it runs into trouble as a unit. If your question is what is this thing we have actually like, that is a composite question.

Synastry is the two of you. A composite is the third thing in the room.

Which to use

Almost everybody arrives with a synastry question and does not know it.

Why does he go quiet when I need him. Why do we have the same argument every autumn. Why am I always the one who reaches out. Why does she read my honesty as an attack. All of those are about what two specific people do to each other, and every one of them is answered by contacts, not by a composite.

Composites earn their keep later, usually for couples who are already stable and are asking a different sort of question: what is this relationship for, what does it want to become, why does it keep pointing us at the same part of life.

One honest caveat about composites

A composite chart is a mathematical construct rather than a moment in the sky. Nothing was ever at those positions. Astrologers disagree, sometimes sharply, about how much weight it deserves, and there is a reasonable case that it tells you more about the astrologer than about the couple.

Synastry has no such problem. Both charts are real skies at real moments, and the angles between them are simply arithmetic. That is part of why it is the older and more widely trusted technique, and why it is the one worth learning first.