Knowing a person's chart, an astrologer can sketch the psychological profile, the mental potential and the program of life of an individual. But if you compare two charts, you can detect some astral relationships that put their imprint directly on the nature of the relationships that can be established between those two persons. This application of astrology is also known under the name of relational astrology.
Many people turn to comparative studies of the charts to find out, for example, if the person they will marry is the most suitable one, or what are the chances for a relationship to be happy. Parents can learn how to better understand their children, etc. In general, any inter-human relationship can be studied from the perspective of relational astrology.
There are several methods for studying the astrological relationships, but the most popular one is the so-called sinastry. The two charts are overlapped so that the zodiac signs coincide, and then there are explored the aspects formed by the planets from a chart with the planets from the other chart. The nature of these issues gives important information on the characteristics of the relationships between those two people and can provide good benchmarks for the adoption of an attitude that will determine a positive evolution of the relationship.
The most popular aspect of synastries is known as the compatibility between zodiac signs, for example when it says "Leo gets along well with Sagittarius." Between Leo and Sagittarius there is a trine on the zodiac, a beneficial and constructive aspect, but perhaps the Sun in Leo of one person is not in trine with the Sun in Sagittarius of the other one.
Another method of the relational astrology is the composite chart. The planetary positions are calculated as the average of the positions from the two charts and thus we determine a medium chart which can give valuable information about the image and the evolution of the couple as a whole, as one being.
In conclusion, it is good to remember that the compatibility of the zodiac signs is a correct theoretical concept, but refers to zodiac signs as a general vibration, not to all natives of those signs. To determine the compatibility of two individuals for a couple life – or, in general, for coexistence and cooperation – is more useful a detailed study of their charts, by using the methods of relational astrology.