The Most Widely Used House System
Placidus is the most commonly used house system in modern Western astrology, developed in the 17th century by the Italian monk Placidus de Titis. It divides the sky using time — the semi-arc method, which takes the diurnal motion of the Sun as its basis. Houses are unequal in size, varying significantly by latitude. At extreme latitudes (above 66°), Placidus breaks down — intercepted signs become common and some houses can span more than 60°.
Placidus vs Other Systems
| House System | Basis | House Sizes |
|---|---|---|
| Placidus | Time / semi-arc | Unequal |
| Equal House | Ascendant + 30° each | Equal (30°) |
| Whole Sign | Sign = house | Equal (30°) |
| Koch | Birthplace time | Unequal |
Using Placidus in Practice
Placidus works well for latitudes between 30° and 60°, which covers most of the inhabited world. Its strength is in producing intercepted signs and unusually large or small houses that reflect the actual astronomical geometry at a birthplace. Most astrology software defaults to Placidus. If you practice at extreme latitudes or work with Hellenistic techniques, you may prefer Whole Sign or Equal House. Experiment with multiple systems on your own chart before committing to one.