A composite chart treats your relationship as if it was its own entity or a third "person" (which in many ways is true!).
In addition to the astrology birth chart, you can do an astrology compatibility chart to see how you'll get along with other people. Synastry charts compare and contrast the planets in your chart and the other person's while a composite chart averages out your two charts using a "midpoint method." It creates a single, blended chart that reveals the essence of your relationship. A composite chart treats your relationship as if it was its own entity or a third "person" (which in many ways is true!).