The seventh house is the seventh of the twelve houses of the horoscope, and one of the four angular houses, the strongest kind. It begins at the descendant, the point of the zodiac setting in the west, directly opposite the ascendant, and it governs partnership in all its forms, marriage, close relationships, and also open enemies and those one deals with face to face. Where the first house is the self, the seventh is the other. As an angular house it lends planets placed in it much force. These are the interpretive conventions of the astrological tradition for this division of the chart, in which the seventh house describes the significant others a person joins with or contends against; it lies across the horizon from the first house, the axis of self and other that runs through the whole wheel.
The Hellenistic Greek name for the seventh house, Dysis, the setting place, mirrors the first house's Horoskopos across the horizon, the point where the zodiac disappears below the western edge of the world just as the ascendant marks where it rises in the east. Vettius Valens, writing around 175 CE, already treats the twelve-place scheme this name belongs to as traditional, and Paulus Alexandrinus set it out systematically two centuries later in 378. No planet was assigned a joy at the descendant, unlike several neighbouring houses, and the earliest layer of the tradition reads the seventh chiefly through its opposition to the first, the self, as the house of the open adversary and the legal opponent as much as the marriage partner, a starker reading than the partnership-centred house most descriptions now give it.
Sources A History of Western AstrologyNicholas Campion with Introduction to Astrology (Eisagogika)Paulus Alexandrinus