House system

Definition

A house system is the mathematical method that splits the zodiac wheel into twelve sectors (the houses) around a natal chart. Astrology has developed several rival systems over its history, each with its own geometric rule for placing the intermediate cusps: Placidus, Koch, Regiomontanus, Porphyry, equal houses, and whole-sign houses are the ones most widely used in modern practice. The choice of system affects the placement of the non-angular houses most of all.

In context

If your chart cast in Placidus places Mercury in the ninth house and Venus in the tenth, a version in equal houses may push one of those two planets into the neighboring house. The four angular houses (one, four, seven, and ten) stay anchored to the Ascendant and Midheaven and therefore do not shift with the system you pick. What shifts is the share of houses two, three, five, six, eight, nine, eleven, and twelve, which is why modern astrology always notes the system used when sharing a natal chart.

To go deeper

A house system belongs to the structural machinery of the chart:

• Whole-sign houses: simple classical system.
• Equal houses: twelve identical thirty-degree sectors.
• Cusp: the degree where each house begins.