7th House
The seventh house is the house of partnership: marriage, business alliances, and open enemies. Its cusp is the Descendant, the point opposite the Ascendant.
What the House Rules
The seventh house rules one-on-one relationships: a spouse, a business partner, a lawyer, and, in the classical tradition, open enemies too, that is, everyone a person deals with face to face.
It is the mirror of the first house: what a person doesn't see in themselves, they meet in a partner.
Planets in the House
Venus in the seventh house strengthens the theme of marriage, Saturn indicates a late or serious union, Uranus unconventional and sudden relationships, Mars conflict or a combative partner.
Planets here more often describe the kind of people a person attracts than the person themselves.
Sign on the Cusp and an Empty House
The sign on the Descendant describes the type of partner a person finds attractive.
An empty seventh house is very common and does not mean being alone. In synastry, two people's charts are compared as a whole rather than looking at a single house.