There are many websites that offer astrological analysis of relationships, including compatibility tests. If each partner's birth date is known, DarkstarAstrology.com has a compatibility test that ranks the strength of relationships based on planetary aspects. Also, HoroscopeCompatibility.com offers a chart that lets users check compatibility by sun sign alone.
Astrological interpretation of relationship charts is called synastry. In synastry, people are considered compatible if the angle between their respective planets is 'soft', meaning it is 60 or 120 degrees. If the angle between their planets is 45, 90 or 180 degrees, astrologers expect tension between the partners. Online astrological sites that ask users to enter both partners' birth dates calculate the angles between their respective planets, and judge the relationship accordingly. Hard aspects, or signs of friction between people, often cause initial attraction that leads to acrimony.
Compatibility charts based on sun sign alone consider the sun sign's element and quality. Some elements and qualities do not mix well. For example, fire and water signs are not compatible. Furthermore, certain signs enjoy change, while other signs loathe change. Astrology concludes that these signs cannot live happily together without considerable effort to resist their own natural inclinations.
DarkstarAstrology.com and most astrologers admit that the usefulness of compatibility tests is limited by the 'weird magic' of human relationships.