The seventh house is representative of a person's interaction with society, one's close associations and one's dealings with the public. Marriage, divorce, business partnerships and close personal relationships belong to this house. Open enemies, rivals, competitors and allies come under the dominion of the seventh house. This house rules agreements, mergers, lawsuits and contracts. Note that while the paper, writing and signing of a contract are third house matters, the essential agreement is a seventh house affair.
The seventh house rules the partner or mate, partnerships in general, marriage, divorce, and open enemies. It is the house of one's opposite, of "the other" that both repels and attracts, entices and frightens. It is the door of knowledge gained by experiencing events not of one's own making. It is the chief entry point of karma. It has to do with board rooms, council chambers, courtrooms, lawyers, and judges. Lots of planets in the seventh house means lots of relationships, lots of trips to court, lots of ups, lots of down, lots of everything. The natural sign of the seventh house is Libra; the natural ruler is Venus.
The Descendant is the back door to our chart-it's where we attract and interact with other individuals. Just as the Ascendant can operate as a kind of a mask that changes both how others see us and how we view the world, the Descendant shows what we look for in partners (primarily romantic partners, of course, but not exclusively). The Descendant is exactly opposite the Ascendant, and it makes sense that the qualities we are the most attracted to in others are the qualities that we feel we ourselves are missing, because we don't see or express them through our Ascendants. Whenever the Descendant is involved, we're relating to other individuals on a one-to-one basis.