Synastry compares two birth charts directly: how one person's planets fall in the other's houses and aspect the other's planets. It shows how each person experiences the other.
A composite chart does something different. It takes the midpoint between each pair of planets and angles to create a single chart for the relationship itself, as if the partnership were a third entity.
This explains a familiar puzzle: two people who get along individually but create something difficult together, or the reverse. Synastry describes the people; the composite describes what they build.
Read the composite Sun for the relationship's purpose, the Moon for its emotional atmosphere, and the Ascendant for how the pair appears to others. A composite Saturn on an angle often describes a serious, lasting bond with real duties.
Composite charts are sensitive to birth times, because the angles and houses depend on both. When one time is uncertain, rely on the planets and their aspects rather than the houses.
Use both methods together. Synastry tells each person what they bring and feel; the composite tells them what the relationship asks of them both.