What a composite chart is
A composite is not built from the moment two people met. It is built from their two natal charts: every planet is taken as the midpoint between the same planets of both partners. The result is a third chart read as a portrait of the relationship itself rather than of either person. Its Sun shows what the pair is held together by, the Moon the everyday climate, Venus and Mars the way they come close and the way they argue.
Angles and houses
Houses are derived from the averaged ARMC of the two charts, so exact angles require the birth time of both people. If a time is unknown, tick the option: the planets stay valid, but the houses and the ascendant of such a chart should not be read.
Composite, Davison and synastry
Synastry compares two charts and shows how the people act on each other. A composite merges them into a single chart of the relationship. A Davison chart does the same through the midpoint in time and space, so it has a real moment and place. The fullest picture comes from reading synastry and composite together.
How to read the result
Start with the composite Sun, Moon and ascendant, then look at the aspects between them. Hard aspects in a composite are not a verdict on the relationship but the places where the pair spends the most effort. A planet without aspects usually marks a theme the two of them rarely live out together.