One of the most frequently asked questions by astrology students is this: which house system is correct? Placidus, Whole Sign, or Regiomontanus? This may actually be the wrong question to ask. Because the "correct" house system depends on what you want to measure. Each system is based on a different geometric principle and answers a different question. In this article, we will examine six fundamental house systems along with their geometric logic.
Before we begin, let us clarify this: there are two things common to all house systems. First, the ecliptic (the Sun's annual path) is divided into 12 sections. Second, the starting points of these sections (cusps) are related to the observer's local horizon and meridian. The difference lies in how this relationship is established.
The Fundamental Question
To design a house system, you need to make three decisions. First: which reference plane to divide (the ecliptic, the celestial equator, the horizon plane, or the prime vertical?). Second: how to divide this plane (equal parts, time-based, or space-based?). Third: how to project the divisions onto the ecliptic (along which great circle?).
Different combinations of these three decisions give rise to different house systems. Let us now examine each one in turn.
Seven Systems
The oldest and simplest system. The entire sign where the ASC falls is the 1st house. The next sign is the 2nd house, the one after that is the 3rd house, and so on. Each house is exactly 30 degrees and coincides with one sign. No calculation is required: knowing the sign of the ASC is sufficient.
In Whole Sign, the MC is not a house boundary. The MC usually falls within the 9th, 10th, or 11th house and is treated as a separate point. This is the most surprising feature for modern astrologers: the MC does not always have to be the 10th house cusp.
The vast majority of Hellenistic authors (Valens, Dorotheus, Firmicus) used this system. It has been repopularized since the 2000s thanks to Robert Hand, Chris Brennan, and the Project Hindsight movement.
The ASC point is the beginning of the 1st house. From here, every 30 degrees along the ecliptic marks the start of a new house. It is similar to Whole Sign but is not bound by sign boundaries: if the ASC is at 15 degrees Gemini, the 1st house extends from 15 degrees Gemini to 15 degrees Cancer.
Like Whole Sign, in Equal House the MC does not have to coincide with the 10th house cusp. It has no issues at high latitudes. The instructions Ptolemy gives for house calculation in the Tetrabiblos are consistent with Equal House, although this is a debated interpretation.
The first quadrant system. It keeps the ASC and MC fixed and divides the ecliptic arc between them into three equal parts. The four quadrants (ASC to IC, IC to DESC, DESC to MC, MC to ASC) are each divided into three equal parts.
It is the most geometrically intuitive quadrant system. It does not use a spatial reference plane; it simply performs arithmetic division on the ecliptic. This simplicity is both its strength and its weakness: it does not rely on a physical rationale but works without issues at every latitude.
The dominant system of medieval Islamic astrology. It divides the diurnal arc (daytime arc) and the nocturnal arc (nighttime arc) into three equal time segments each. It then projects the celestial equator equivalents of these time segments onto the ecliptic.
Alcabitius moves Porphyry's ecliptic division to the celestial equator. The difference is this: Porphyry divides ecliptic degrees equally, while Alcabitius divides time equally. Since equal time on the equator equals equal degrees, Alcabitius effectively divides the celestial equator into equal parts and projects them onto the ecliptic.
Abu Ma'shar, al-Kindi, Masha'allah, and Ibn Ezra used this system. Bonatti also states that his preferred system is Alcabitius. It remained the most widely used quadrant system in Europe until the 15th century.
It divides the celestial equator into equal 30-degree parts (like Alcabitius), but differs in its projection method. Regiomontanus projects the house boundaries onto the ecliptic along horizon-meridian planes (house circles).
This is the system William Lilly preferred for horary astrology and it is considered standard in the English horary tradition. It is also used in natal astrology, but serious distortions can occur at high latitudes.
It divides the observer's head-to-foot plane (prime vertical) into equal 30-degree parts and projects these onto the ecliptic. Unlike other quadrant systems, it uses the local spatial plane as its reference rather than the celestial equator.
This is the feature that makes Campanus the most physically meaningful system: it truly divides the space around the observer into equal parts. However, at high latitudes, practical problems can arise because the angle at which the prime vertical intersects the horizon narrows.
The most widely used system in Western astrology today. It divides the time from a planet's rising to the MC and from the MC to its setting (semi-diurnal and semi-nocturnal arc) into three equal time segments each. The starting and ending points of each time segment on the ecliptic form the house cusps.
Placidus, like Alcabitius, performs a time-based division, but its projection method is different. While Alcabitius projects celestial equator time segments using hour circles, Placidus calculates each point's own diurnal arc ratio. This requires an iterative calculation and is the most difficult system to compute by hand. Its widespread adoption came when Raphael published ready-made house tables in the 19th century.
Placidus's greatest weakness is its inability to function at high latitudes (65 degrees and above). Some signs never rise or set, which means house cusps cannot be calculated. For this reason, Placidus cannot be used in Scandinavian countries or northern Canada.
Comparison Table
| Feature | Whole Sign | Equal | Porphyry | Alcabitius | Regiomontanus | Campanus | Placidus |
|---|---|---|---|---|---|---|---|
| Divided plane | Ecliptic (signs) | Ecliptic | Ecliptic quadrants | Celestial equator | Celestial equator | Prime vertical | Semi-diurnal arc |
| Division method | Sign boundaries | 30 degrees equal | Quadrant trisection | Time equalization | 30 degrees equal + meridian projection | 30 degrees equal + horizon projection | Semi-arc time trisection |
| ASC = 1st house cusp | No (sign start) | Yes | Yes | Yes | Yes | Yes | Yes |
| MC = 10th house cusp | No | No | Yes | Yes | Yes | Yes | Yes |
| Houses equal in size | Yes (30 degrees) | Yes (30 degrees) | No | No | No | No | No |
| High latitude issue | None | None | Minimal | Yes | Yes | Yes | Severe (65 degrees+) |
| Can intercept occur | No | No | Yes | Yes | Yes | Yes | Yes |
| Historical usage | Hellenistic | Hellenistic | Late antiquity | Islamic medieval | Late medieval | Late medieval | 17th century+ |
| Modern usage | Traditional astrology | Vedic, some Western | Limited | Limited | Horary | Limited | Most common (Western) |
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Practical Differences
The Role of the MC
In quadrant systems (Porphyry, Alcabitius, Regiomontanus, Campanus), the MC is always the beginning of the 10th house. In Whole Sign and Equal House, however, the MC is an independent point and can fall within the 9th, 10th, or 11th house. This distinction has a major practical consequence: for someone whose MC falls in the 9th house, the "career house" in a quadrant system is the 10th house, while in Whole Sign it is the 9th house. Especially if the planets in these houses differ, the interpretation changes entirely.
House Sizes
In Whole Sign and Equal House, every house is exactly 30 degrees. In quadrant systems, however, houses are not equal in size. Particularly when the angle between the ASC and MC differs significantly from 90 degrees (common at high latitudes), some houses can be 40-50 degrees while others are only 15-20 degrees. This means some signs can be entirely "trapped" (intercepted) within a house. How to interpret intercepted signs is a matter of debate.
The Difference Between Alcabitius and Regiomontanus
Both divide the celestial equator, but their projection methods differ. Alcabitius projects house boundaries onto the ecliptic along hour circles. Regiomontanus uses house circles (horizon-meridian planes). At mid-latitudes the difference is small (1-3 degrees), but it becomes more pronounced at high latitudes. In horary astrology, Regiomontanus is considered standard; in natal astrology, Alcabitius was preferred in the Islamic tradition.
Practical Tip
If a planet is near a house boundary (within 3 degrees of the cusp), it may fall into different houses in different systems. In such cases, rather than committing to a single system, check multiple systems. If the planet is clearly on one side of the cusp (5 degrees or more), the system difference generally does not change the result.
Which One Should You Use?
There is no definitive answer to this question, but some guidance can be offered.
If you are studying Hellenistic astrology and reading Valens, Dorotheus, and Firmicus, Whole Sign is the most consistent choice. Most of the techniques of these authors (profections, lots, time-lord systems) were designed with Whole Sign in mind.
If you are studying medieval Islamic astrology (Abu Ma'shar, al-Kindi, Ibn Ezra, Bonatti), Alcabitius is the primary choice. You will also need to know Regiomontanus for horary questions, as the Lilly tradition is based on this system.
If you are studying modern psychological astrology, Placidus is the most common standard. It became standard thanks to the ready-made house tables Raphael published in the 19th century. However, keep in mind that it has serious calculation issues at high latitudes and that authors before the 17th century did not use this system.
The house system is not what determines whether astrology is "right" or "wrong." Whether a chart is meaningful depends on the astrologer's understanding and consistency. What matters is choosing a system and learning it in depth.-- Sira Nur Uysal
The Two-Layered Approach
Some modern traditional astrologers recommend a two-layered approach: Whole Sign for planetary house placement, and a quadrant system (usually Porphyry or Alcabitius) for assessing whether planets are angular, succedent, or cadent. This approach is also largely consistent with Hellenistic practice: certain passages in Valens suggest that he thought in quadrant terms when assessing angular strength, but determined house topics according to Whole Sign.
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