Astrolium's house systems guide covers the canonical Placidus-versus-whole-sign argument plus the 21 other systems Astrolium ships, with a decision tree for picking the right system for your work. Robert Hand on Placidus drift at high latitudes; Project Hindsight on whole sign's return; the working astrologer's view on when to switch.
For the underlying feature with all 23 systems in 1 click, see house systems. For the historical case for whole sign as the default, read why we kept whole sign houses. For the related horary work where Regiomontanus is canonical, see horary astrology. For the $29 per month Pro plan with all 23 systems, see pricing.
What a house system actually does
There are about 30 named house systems in the historical record. Astrolium ships 23, covering the full Western tradition and the closest Vedic equivalents. The two systems most working astrologers actually choose between are whole sign and Placidus; the others have constituencies but are minority positions in contemporary practice.
Three reasons house systems matter. One: the same planet at the same zodiacal degree can fall in different houses across systems, which changes the reading. A Mars at 28° Pisces in a Capricorn-rising chart sits in the 3rd house in whole sign and the 2nd or 4th in Placidus depending on the latitude. Different houses, different readings. Two: predictive techniques walk through the houses, so the system determines which year is what. Three: a chart published in one system is not reproducible by a reader using another unless the system is labelled, which is why Astrolium names the active system in every export.
Whole sign explained
In the whole sign system, each sign is one house. If your Ascendant falls at 14° Cancer, then Cancer (all 30° of it) is your first house, Leo is your second, Virgo your third, and so on. The cusps fall on sign boundaries. Houses don't get larger or smaller depending on latitude. The math is trivial; the implications are deep.
This is the system used by every Hellenistic astrologer whose work survives. Vettius Valens uses it throughout the Anthology. Dorotheus uses it in the Pentateuch. Ptolemy uses it (on most readings) in the Tetrabiblos. The medieval Persian and Arabic tradition that inherited the technique used it. It was the default for at least 1000 years of recorded astrological practice before quadrant systems became dominant in the Latin West.
The technique returned to mainstream Western practice through Robert Schmidt and the Project Hindsight translation initiative of the 1990s. Schmidt's reconstruction of Hellenistic predictive astrology (profections, zodiacal releasing, sect doctrine) surfaced the fact that these techniques were developed in a whole-sign world. Running them against Placidus houses introduces small but real distortions in which planet rules which year.
Robert Hand wrote a short monograph (Whole Sign Houses, ARHAT) that articulates the historical case as clearly as anyone has. The summary: whole sign is older, simpler, more historically grounded, and works at every latitude. The arguments against it are usually arguments for angular precision, which whole sign sacrifices in exchange for everything else.
Placidus explained
Placidus is a quadrant system: it divides the visible day-arc and night-arc of the ecliptic into 6 equal time portions per quadrant. The cusps fall wherever those time-divisions land on the ecliptic, which means at high latitudes the houses get extremely lopsided. 10° here, 50° there.
The geometry is detailed. Take the Ascendant and the MC; trisect each of the 4 quadrants of the diurnal arc by equal time (not by equal space); project the resulting points onto the ecliptic. The result is a house system encoding the diurnal motion of the ecliptic at the birth location, which lets you run certain time-based predictive techniques (primary directions, in particular) cleanly.
Placidus was published by Placidus de Titis, an Italian Catholic monk, in 1657, ten years after Lilly's Christian Astrology came out. It was popularised in the English-speaking world by Raphael's Ephemeris and the standard 19th and 20th-century table books, which printed houses in Placidus by default. Once the printed tables existed, switching was hard, and Placidus became the de facto 20th-century standard.
Why the 20th century picked Placidus
Three reasons, none of them about astrological theory.
One: the printed tables. Raphael's Ephemeris (continuous publication from 1822) and the standard American Ephemeris published houses in Placidus. To compute houses in another system, you had to either own a different table book or do the math by hand, which most working astrologers were not equipped to do. The Placidus default was an artefact of publishing, not a deliberate choice.
Two: angular precision. Placidus gives finer angular distinctions than whole sign. A planet at 2° from the MC sits in the 9th or 10th house depending on which side, where whole sign assigns it by sign regardless. Some practitioners valued the angular precision enough to accept the latitude trade-off.
Three: primary directions. The 19th and early 20th-century predictive method of choice was primary directions, which were developed in a Placidus frame and read most cleanly there. Vivian Robson, Sepharial, and the standard British astrology texts of the era assumed Placidus throughout.
The 1990s changed all three conditions. The Project Hindsight translations brought the Hellenistic predictive methods back into circulation. Computer software made any house system trivially available. The printed-tables monopoly evaporated. The case for Placidus weakened in proportion.
Why working astrologers are returning to whole sign
Five reasons, all of them substantive.
One: the predictive techniques. Profections, zodiacal releasing, time-lord work, and most of the surviving Hellenistic predictive methods were developed in a whole-sign frame. Running them against Placidus introduces small but real distortions.
Two: global practice. Astrologers with clients above 60° N (Stockholm, Helsinki, Tromsø, Reykjavík) or below 45° S (Wellington, Hobart, Punta Arenas) find Placidus houses unstable and sometimes unreadable. Whole sign produces a usable chart at every latitude on Earth.
Three: simplicity. A beginner reads whole sign more easily than Placidus because each sign is one house. The reading is structurally cleaner.
Four: the historical case. Whole sign is the older system and the system most of the surviving classical texts assume. For practitioners who want to read in continuity with the tradition, this matters.
Five: the audit trail. A whole-sign chart is reproducible from the Ascendant alone. A Placidus chart requires the full computation, which means a reader needs the same software to verify. Whole sign charts are easier to publish and easier for other practitioners to check.
The trade-off is angular precision. A planet at 1° from the MC is in the 10th house in whole sign regardless of which side, while Placidus puts it in the 9th or 10th depending on the geometry. Some readings benefit from that distinction. Most don't.
The high-latitude problem
Robert Hand's writing on house systems is the canonical treatment of this issue. Placidus is built on the diurnal motion of the ecliptic, meaning the time it takes for a degree to rise from the horizon, culminate at the MC, and set on the western horizon. Above 66° latitude (the polar circle), some degrees of the ecliptic never rise or never set during certain weeks of the year. The diurnal arc becomes mathematically undefined and the house divisions cannot be computed.
This is not a numerical glitch. The mathematics genuinely break. Software that claims to compute Placidus above 66° is either using a fallback approximation (which produces nonsense house positions) or returning garbage. Astrolium flags any chart above 66° N or below 66° S with a warning and offers whole sign, equal, or Porphyry as drop-in replacements.
Between 60° and 66° N or S, Placidus and Koch still compute but produce houses that vary in size by a factor of 5 or more. A 50° first house against a 10° third house is technically correct in the math but practically unreadable for client work. Whole sign keeps each house at 30° regardless of latitude.
Working astrologers in Helsinki, Tromsø, and Reykjavík uniformly settle on whole sign or equal for this reason. The Placidus tradition was developed and tested in Mediterranean latitudes; it does not scale to the polar circle.
Decision tree: how to choose
Five questions. The answer to most charts comes out in 2 or 3 of them.
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What tradition do you read in? Hellenistic, traditional, or Project Hindsight-influenced: whole sign. 20th-century English-language sources or Western tropical mainstream: Placidus. German tradition: Koch. Medieval horary: Regiomontanus. Uranian: Meridian. Vedic-influenced Western: Sripati.
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What latitude are your clients at? Above 60° N or below 60° S, quadrant systems become unstable. Above 66°, Placidus and Koch fail mathematically. Whole sign, equal, or Porphyry work everywhere on Earth.
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What technique are you running? Predictive Hellenistic techniques (profections, zodiacal releasing) were developed for whole sign and read most cleanly there. Primary directions were developed for Placidus and read most cleanly there. Horary is canonically Regiomontanus.
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Has the client been read in another system before? Once a client has a written reading in Placidus, switching to whole sign mid-relationship breaks continuity. Astrolium lets you set the house system per client, not just globally.
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What does the chart look like in each? When unsure, render the chart in 2 or 3 systems side by side. The system that produces a chart you can actually read is the right system for that chart. Astrolium's compare mode renders 2 wheels in 160 ms total.
Porphyry, Equal, Koch, Regiomontanus
Porphyry trisects the quadrants by equal arc on the ecliptic. The simplest quadrant system. Documented by Porphyry of Tyre in the 3rd century CE and listed in some earlier Hellenistic sources. A reasonable default for practitioners who want the angles of a quadrant system without the latitude problems of Placidus.
Equal house assigns 30° to each house starting from the Ascendant. Houses can split signs. Avoids the latitude problem of Placidus while preserving angular precision against the Ascendant degree. Some Project Hindsight-era practitioners use equal as a hybrid: whole-sign signs for the houses, equal-house cusps for angular planets.
Koch divides the diurnal arc at the birth point. Walter Koch published it in 1962; popular in 20th-century German practice and still common among practitioners trained in the German tradition. Mathematically similar to Placidus, with similar failures at high latitudes.
Regiomontanus divides the celestial equator into 12 equal arcs and projects them onto the ecliptic. The medieval Latin standard before Placidus took over in the 17th century. The horary default for most working horary practitioners; Astrolium's horary feature defaults to Regiomontanus accordingly. Named for Johannes Müller von Königsberg ("Regiomontanus" being the Latinised form), a 15th-century mathematician.
Campanus, Alcabitius, Morinus
Campanus divides the prime vertical (the great circle running east-up-west through the celestial sphere) into 12 equal arcs. Geometrically the most elegant of the quadrant systems; rarely used in working practice. Named for the 13th-century scholar Giovanni Campano da Novara.
Alcabitius divides the Ascendant-to-MC arc into 3 time portions per quadrant. The medieval Arabic standard, inherited by some Renaissance Latin authors. Named for the 10th-century Arabic astrologer al-Qabīṣī. Used by some contemporary Arabic-tradition practitioners.
Morinus projects equal arcs of the equator onto the ecliptic without quadrant constraints. A 17th-century system by Jean-Baptiste Morin de Villefranche, the French Catholic astrologer who wrote Astrologia Gallica. Used by a small group of contemporary practitioners, particularly the French-tradition descendants.
Sripati and the Vedic question
Sripati is the closest Western equivalent to the Vedic Bhāva Chalit system. The cusps fall in the middle of the rāśi house spans rather than at the boundaries: a hybrid between whole sign (each sign is one house) and a quadrant system (cusps at sensitive points within the sign).
For Western practitioners working with Vedic clients or Vedic-influenced material, Sripati is the right Western proxy. The full Vedic chart engine (rāśi, navāmśa, daśās, the panchanga) is on the Astrolium roadmap for V1.2. Until then, Sripati covers the bhāva positions while the rāśi chart stays in whole sign.
The tropical-versus-sidereal question is separate from the house systems question and is supported across all 23 of Astrolium's house systems. Pick your house system and your zodiac independently.
When to switch mid-reading
Three situations where switching house systems on a single chart produces a useful read.
An angular planet within 2° of a cusp is the first such case. Whole sign and Placidus may disagree about which side of the cusp the planet sits on. Render both, compare to the client's life, and pick the system whose assignment matches the lived experience. The chart often votes for itself once you see both views.
The second case is a predictive technique conflict. You're running profections in whole sign (the canonical frame) but want to check primary directions on the same chart. Switch to Placidus for the directions, switch back to whole sign for the next profection. Astrolium remembers per-technique settings.
The third case comes up when speaking with another practitioner. A colleague reads in Placidus and sends you a chart for a second opinion. Open it in their system to read the same chart they're reading. Then optionally switch to your default to see what your system shows. Different lenses on the same chart.
Common mistakes
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Assuming the planets change. They don't. A planet at 14° Cancer is at 14° Cancer in every house system. What changes is which house it falls into, not where it is in the zodiac.
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Publishing without the system in the metadata. A chart published in Placidus that doesn't say so is unreadable to a whole-sign practitioner trying to verify it. Astrolium names the system in every export by default.
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Treating one system as "correct." The systems answer different questions. Whole sign answers "what sign is foregrounded." Placidus answers "what time-arc is foregrounded." They are different lenses on the same chart, and a working astrologer benefits from being able to switch.
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Switching systems mid-relationship without telling the client. A client who has been read in Placidus for 5 years expects their chart to look a certain way. Switching to whole sign produces a different chart and breaks continuity. If you switch, explain why and walk the client through both readings.
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Using Placidus above 66° latitude. The system fails mathematically. Use whole sign, equal, or Porphyry instead.
What to read next
For the underlying Astrolium feature with all 23 systems in 1 click, see house systems. For the historical case for whole sign as the install default, read why we kept whole sign houses. For the related horary work where Regiomontanus is canonical, read horary astrology and the horary feature. For Robert Hand's monograph on the topic, Whole Sign Houses (ARHAT, 2000) is the working text. For the practitioner-level treatment of Placidus, Vivian Robson's The Radix System and any of the standard 20th-century English-language astrology textbooks (Margaret Hone, Jeff Mayo, Sepharial) ground the case.
