The astrological technique of comparing two individuals' birth charts for compatibility is called synastry. In general  I will look at the positions and influences of the planets in both charts, and then look at the relationship between each planet in one chart to each planet in the other chart. Just as with a single chart,  I look at the number of degrees, which separate the planets - but across the charts. If, for example, your Venus makes an aspect to your partner's Mars, there is probably a very strong sexual attraction between the two of you.