House systems, and which one to choose
A house system is a way of cutting a chart into twelve. There are many, they disagree, and the disagreement is not a mistake — they are answers to slightly different questions about what a house is. This site offers seven.
The short answer
Choose Placidus if you have no reason to prefer another. It is the modern Western default, it is what an astrologer will assume you used, and it is what most books published in the last century are working from.
Choose whole sign if you are reading traditional or Hellenistic material, where it is the native system and where quite a lot of technique depends on it.
Choose equal houses if you were born at a high latitude and want something that behaves the same way everywhere.
What changes when you switch, and what does not
Switching systems never changes which sign a planet is in, or any aspect between planets. Those come from the planets’ positions, which no house system touches. What changes is the house numbers — the same planet can be in the eighth house under one system and the ninth under another — and how wide the houses are.
So if two charts disagree about a Sun sign, the house system is not why. Four things can cause that, and the birth time is usually the one.
The seven, and what each divides
| System | What it divides |
|---|---|
| Placidus | Time. Each cusp is the degree that has spent a fixed fraction of its own rising-to-culminating time on the way there. |
| Whole sign | Nothing. Each sign is a house, the rising sign is the first, and no division happens at all. |
| Equal | Ecliptic longitude, in twelve exact thirty-degree steps from the rising degree. |
| Koch | Time as well, but measured from the birthplace's own horizon rather than the equator. |
| Porphyry | Ecliptic longitude. The arcs between the four angles are simply cut into three. |
| Campanus | Space, along the prime vertical — the great circle running east, overhead, west. |
| Regiomontanus | Space, along the celestial equator, divided into equal arcs. |
The four that divide time or space — Placidus, Koch, Campanus, Regiomontanus — put the Ascendant on the first cusp and the Midheaven on the tenth, and produce houses of unequal width. The two that divide longitude keep the geometry simpler. Whole sign does no dividing at all, which is why it is the one system that cannot break.
Where they stop working
Above about 66.5 degrees of latitude — the Arctic and Antarctic circles — some of these systems become undefined rather than merely awkward. Placidus and Koch divide the time a degree of the ecliptic spends between rising and culminating, and inside the polar circles there are degrees that never rise or never set, so there is no such span to divide. Campanus and Regiomontanus can produce cusps that run backwards around the circle, which is faithful to the geometry and useless for deciding which house a planet is in.
This site does what Swiss Ephemeris does and substitutes Porphyry in those cases — and, unlike most, says so on the chart: the page tells you which system your cusps actually are, which one you asked for, and why they differ. If you were born in Tromsø or Utqiagvik, that note is the honest part of the chart.
Whole sign and equal houses are defined at every latitude short of the pole itself, which is a practical argument for either of them if you were born far north.
Is one of them correct?
No, and anyone who tells you otherwise is telling you which tradition they were trained in. The systems have been argued over for two thousand years and the argument is about what a house is — a division of space, of time, or of the zodiac itself. What you can reasonably do is pick one, read with it consistently, and know which one you picked.
Calculate your chart in any of the seven; the house system is the last field on the form, and it is in the address of the result, so you can change it and compare.