House systems: why Placidus and whole sign give different answers

House system · default Placidus · 6 systems available · Placidus undefined above 66° latitude, falls back to Porphyry

01The same sky, six different rulers

Every house system starts from the same inputs: the Ascendant, the Midheaven, and the latitude and longitude of the birth. What differs is the rule each system uses to carve the remaining space between those angles into twelve houses. No system moves a single planet. A Mars at a particular ecliptic longitude sits at that longitude under every house system a chart could be run with — what changes from one system to the next is which house that fixed degree happens to fall inside, because the boundaries drawn around it move even though the degree itself does not.

Lizastro computes six systems from the same underlying chart data, lets a reader switch between them, and stamps every chart with which one produced the house placements shown, because the difference between systems is real enough that it is worth being explicit about rather than assumed.

02Placidus, the default

Placidus divides the sky by time rather than by degree: it takes the time it takes a given point of the ecliptic to travel from the Ascendant to the Midheaven, or from the Midheaven to the Descendant, and splits that duration into equal thirds to place the intermediate cusps. Because it is measuring time along paths that curve differently depending on latitude, the resulting houses are not equal in size — some noticeably wider than thirty degrees, others noticeably narrower, in the same chart. It is the most widely used system in modern practice, and it is Lizastro's default.

The time-based method has one sharp edge: above roughly 66 degrees of latitude — inside the Arctic and Antarctic circles — some points of the ecliptic never cross the horizon at all for parts of the year, at which point the time-based construction has nothing to divide and the system becomes undefined. Lizastro's engine falls back to Porphyry automatically in that case, and says so on the chart, rather than returning a Placidus result that does not actually exist for that birthplace and pretending it does.

03Whole sign, the oldest

Whole sign uses no time-based division at all. Whatever sign the Ascendant falls in becomes the entirety of the first house, cusp to cusp with that sign's own boundaries, and each following sign in order becomes the next house — twelve houses, twelve signs, one each, with no house ever splitting a sign in two. It is the system used in the Hellenistic tradition, older than Placidus by well over a thousand years, and its simplicity is also its main practical advantage: because it depends only on which sign the Ascendant falls in rather than its exact degree, it is far more resistant to an uncertain birth time than any of the other five systems Lizastro offers, all of which depend on the Ascendant's precise degree to place a cusp correctly.

04The other four

Koch divides time the way Placidus does, but it trisects a different span: rather than each cusp's own path from the Ascendant to the Midheaven, it takes the single arc the Midheaven degree itself travels to reach the horizon at the birthplace, and divides that arc into thirds instead. It is common practice across Central Europe in particular. Like Placidus, it produces unequal houses and shares the same difficulty at extreme latitudes, for the same underlying reason: both are built on a time-based division that needs the ecliptic to actually cross the horizon in a well-defined way, which is precisely what stops happening near the poles.

Equal takes the Ascendant's exact degree and lays out twelve equal thirty-degree segments from there, which keeps every house identical in size but means the Midheaven does not necessarily land exactly on the boundary of the tenth house the way it does in the four quadrant systems — the Midheaven is computed independently and simply falls wherever the equal division happens to place it, sometimes inside the ninth or eleventh house rather than exactly on the tenth's cusp. That is not an error in the calculation; it is the direct consequence of building the wheel from one angle only instead of reconciling two.

Porphyry is the oldest of the quadrant systems still in use, older than Placidus and closer in age to whole sign than to any of the other time-based methods: it simply divides the arc from Ascendant to Midheaven, and Midheaven to Descendant, into three equal parts each, without reference to time of travel at all. That construction is simple enough that it stays defined at every latitude with no exceptions, which is exactly why it is the fallback Lizastro's engine uses automatically when Placidus cannot be computed.

Regiomontanus divides the celestial equator rather than the ecliptic into equal segments and projects those divisions back onto the ecliptic to produce the house cusps, a method developed for and still associated with the horary tradition — chart questions asked about a specific moment rather than a birth — more than with natal astrology.

05What actually moves, with numbers

The clearest way to see the difference is a boundary case. Under Placidus, house sizes routinely depart from a clean thirty degrees — an intermediate house might span twenty-two degrees while its neighbour spans thirty-eight, because the underlying division is by time along a curved path rather than by an even split of arc. A planet sitting near one of those uneven boundaries can fall in a different house depending on which system measured it, even though its own ecliptic longitude is identical in both readings. Under whole sign, by contrast, that same planet either falls within the Ascendant's sign or it does not — a question with no boundary case inside a sign, only at the edges between signs, which return to the birth-time sensitivity the rising sign article covers directly.

The disagreement also grows with latitude. Near the equator, the six systems tend to produce fairly similar-looking cusps, because the diurnal arcs the time-based systems measure are closer to symmetrical there. Further from the equator, those arcs become more lopsided between the two halves of the day, which is exactly the condition that makes Placidus's thirds uneven in the first place — so the gap between what Placidus says and what a degree-based system like Equal says tends to widen the further from the equator a birth took place, right up to the point above 66 degrees where Placidus stops being computable at all.

06What it is not

No house system is the correct one that the other five are approximations of. Each is a different, internally consistent answer to the same question — how to divide the space between four fixed angles — and different schools of practice have settled on different defaults for reasons of tradition, region, and technique rather than one being demonstrably more accurate at describing a birth. A chart's planets, signs, and aspects are identical no matter which system computed the houses; what changes is only which house each placement is read through, which is a real difference in emphasis but not a disagreement about where anything actually is in the sky.

It is also not a setting worth hiding behind a fixed default and calling settled. A house system determines which house a given planet is read through, and a reader who cares about the distinction is entitled to see it stated and to change it, rather than discovering only by chance that a tool defaulted to a system it never named.

Nor is house placement the same question as sign placement, even though the two get folded together in casual conversation. A planet's sign is fixed by its ecliptic longitude alone and does not depend on the birth time or place at all beyond the date; a planet's house depends on the birth time, the birth place, and which of six defensible systems was used to divide the space between the angles. Two readers with the same Mars in the same sign can have that Mars in entirely different houses, for entirely legitimate reasons, and neither reading is wrong about the sign — they are answering the house question with different, equally valid arithmetic.

07Where you see it in your own chart

Switching systems on the same birth data is the fastest way to see the mechanism directly: the planets stay exactly where they were, the signs stay exactly where they were, and only the house cusps move, sometimes by enough to carry a placement near a boundary into the house next door. A chart with most of its placements sitting comfortably in the middle of wide houses will barely change appearance between systems at all; a chart with several placements sitting close to a cusp is the one where the choice of system is worth looking at directly rather than accepting a single default.

An empty house, in any system, is not itself informative about how much is going on in that area of a life — most charts have several empty houses simply because ten bodies cannot fill twelve slots, whichever system drew them. What an empty house changes to is the sign on its cusp and where that sign's ruling planet sits elsewhere in the chart, and that answer can itself shift when the cusp moves between systems, which is one more reason the choice of system is worth being deliberate about rather than accidental.

→ Compare house systems on your own chart · Read the full methodology

08Related reading

The rising sign article covers the Ascendant itself in more depth — the number every one of these six systems, whole sign included, is ultimately built from. Capricorn is the sign naturally associated with the tenth house and the Midheaven, the other angle a house system has to place correctly.

Updated 2026-08-12.