What are house systems? Placidus, Equal House, Whole Sign, Porphyry

When you calculate a birth chart, you'll see an option on the form called "House system". Next to it are some unfamiliar names: Placidus, Equal House, Whole Sign and Porphyry. This choice determines which method is used to draw the boundaries of the twelve houses in your chart. Here's the short answer up front: if you're not sure, stick with Placidus. In this article we explain how the four methods work, what they change in your chart and which one to choose in which situation. While you read, you can generate your own chart on the birth chart page and keep it beside you.

What do the houses describe?

A birth chart has two wheels laid on top of each other. The signs describe how a planet behaves, while the houses describe the area of life where that behavior shows up. If Venus is in Taurus, it expresses love in a calm, patient and tactile way. If that same Venus is in the 10th house, this quality shows most clearly in your working life and in how you present yourself in public.

The zodiac is the same for everyone. The houses, however, are built from the horizon at the moment and place of your birth. That's why your date of birth isn't enough to calculate the houses: you also need the time and the city. You can find what each house governs, one by one, in the houses guide.

The four angles

Every house system starts from the same four points:

Ascendant (ASC): The degree rising on the eastern horizon at the moment of your birth.
Descendant (DSC): The degree setting on the western horizon, directly opposite the Ascendant.
Midheaven (MC): The degree at the highest point of the meridian, toward the south.
Imum Coeli (IC): Directly opposite the MC, the point beneath your feet.

These points don't depend on the system you choose. Your Ascendant degree comes out the same whichever method you use. What sets the systems apart is how the space between these points is divided into twelve houses.

Why is there no single correct method?

The sky is a sphere, but a chart is a flat circle. The path the Sun follows through the year, the ecliptic, is tilted about 23.5 degrees to the Earth's equator. Because of this tilt, the signs don't rise over the horizon at the same speed. At Turkey's latitude, some signs rise in about an hour, while others take close to three hours.

There is more than one geometric way to divide the sphere into twelve parts. Some methods are based on time, some on degrees along the ecliptic, and some directly on the signs. Over more than two thousand years, astrologers have given different answers to this question, and the systems in use today are the most widespread of those answers.

Placidus

This is the most widely used system in Western astrology today. It is named after Placidus de Titis, a 17th-century Italian monk and mathematician. Placidus is based on time: it divides the time a degree takes to travel from the eastern horizon up to the Midheaven into three equal parts and places the house cusps according to those intervals. The same calculation is done for the half below the horizon.

This is why the houses come out in different sizes. At Turkey's latitudes, the difference stays within reasonable limits. The farther north you go, the wider some houses become and the narrower others get. Beyond the Arctic Circle, roughly above 66 degrees, some degrees never rise or never set, and the system can no longer be calculated. For someone born at these latitudes, 12astro draws the chart with Porphyry even if Placidus is selected.

There is also a practical reason Placidus became so widespread: in the 19th and 20th centuries, the ephemerides and tables of houses most used in the English-speaking world were printed according to this system. Most of the astrology books and websites you read today are written with Placidus houses. In this system, the MC is always the cusp of the 10th house and the IC the cusp of the 4th.

Equal House

This is the simplest method. The Ascendant degree becomes the cusp of the 1st house, and each following house begins exactly 30 degrees after the previous one. If the Ascendant is at 12 degrees Leo, the 2nd house begins at 12 Virgo, the 3rd at 12 Libra, and so on around the wheel.

Its roots go back to antiquity. In the 20th century it became popular again in England through the textbooks of astrologers such as Margaret Hone. It works the same way at every latitude. The point to watch is the MC: in this system the Midheaven isn't a house cusp, and depending on the place and time of birth it can fall in the 9th, 10th or 11th house. Astrologers who use Equal House read the MC as a separate sensitive point.

Whole Sign

This is the oldest known house method and forms the foundation of Hellenistic astrology. Whatever sign the Ascendant is in, that entire sign counts as the 1st house, the next sign is the 2nd house, the one after that the 3rd. No matter which degree of Leo the Ascendant is in, the 1st house begins at 0 Leo, the 2nd house is Virgo and the 3rd is Libra.

In this method, houses and signs share the same boundaries. To find which house a planet is in, you only need to look at its sign. In the example above, if Mercury is in Libra, it's in the 3rd house. Indian astrology also builds its houses largely on this logic. New translations of ancient texts in the late 20th century revived interest in it in the West, and today it is common among those who work with traditional astrology.

Whole Sign also has a practical advantage: even if your birth time is off by a few minutes, your houses don't change as long as the Ascendant stays in the same sign. Sun sign horoscopes also use a variation of this method, treating the sign the Sun is in as the 1st house. The daily horoscopes on 12astro are written with this logic. If you're curious about the differences between Western and Indian astrology, take a look at our Vedic and Western astrology article.

Porphyry

It takes its name from Porphyry, the 3rd-century Neoplatonist philosopher. It divides each of the four quadrants between the angles into three parts of equal degrees. If the span between the Ascendant and the Imum Coeli is 96 degrees, the 1st, 2nd and 3rd houses are each 32 degrees.

Like Placidus, it uses the four angles as house cusps, and the houses again come out in different sizes. The difference is that the calculation rests directly on degrees rather than on time. That's why it gives a result at every latitude. Because it shares the same four angles with Placidus, its results are often close to it, and it makes a good fallback at northern latitudes where Placidus doesn't work.

The four systems at a glance

System	How does it divide the houses?	Is the MC a house cusp?	In polar regions
Placidus	By time; houses vary in size	Yes	Cannot be calculated
Equal House	30 degrees each, starting from the Ascendant	No	Works
Whole Sign	Each sign is one house	No	Works
Porphyry	Each quadrant in three equal parts	Yes	Works

What does the house system change in your chart?

First, what stays the same: the signs of the planets, the aspects between them, and the Ascendant and MC degrees remain identical whichever system you choose. Two things change: the cusps of the intermediate houses, and which house a planet sitting near a house boundary appears in.

Imagine a chart with the Ascendant at 25 degrees Leo. The Descendant, the cusp of the 7th house, falls at 25 degrees Aquarius. If Venus is at 20 degrees Aquarius, in Placidus it hasn't yet reached the 7th house cusp and appears in the 6th house. In Whole Sign, however, all of Aquarius is the 7th house, so the same Venus is in the 7th. In the first reading, Venus is interpreted within daily work and health routines; in the second, within the area of relationships.

If your chart has planets sitting on a boundary like this, reading the meanings of both houses together usually gives the most balanced result. Some astrologers already count a planet within five degrees of the next house cusp as being in that house. Planets far from a boundary, on the other hand, will most likely stay in the same house whichever system you use.

Which one should you choose?

Placidus if you're new to astrology: You'll be speaking the same language as most of the books, interpretations and websites you read. It's also the default option on 12astro.
Whole Sign if you're unsure of your birth time: As long as your rising sign is correct, your houses aren't affected by small errors in the time. If you have no idea of your birth time at all, you can first try finding the most likely time with the birth time rectification tool.
Equal House or Porphyry if you were born near the Arctic Circle: In places such as northern Norway, Sweden and Finland, Placidus doesn't give reliable results.
Whole Sign if you're interested in traditional or Indian astrology: The techniques in the ancient texts were built on these houses.

The easiest way to see which one suits you better is to try them. Generate the same chart with two different systems and look at the planets that change houses. Whichever reading feels closer to what's happening in your life, stay with that system for a while.

Changing the house system on 12astro

On the birth chart and relocation chart forms, you'll find the "House system" option below the birth details. After the calculation, open the "House cusps" section on the results page to see which degree of which sign each house begins in. Resubmitting the form with a different system and comparing the two results takes just a few seconds.

You can also find short summaries of the house systems and the meanings of the twelve houses in the houses guide. If you don't know where to start interpreting house placements, our article how to read a birth chart is a good starting point.

Frequently asked questions

Does changing the house system change my sign or my Ascendant?

No. The signs of the Sun, Moon and other planets, your rising sign and the MC stay the same. The only things that change are the cusps of the intermediate houses and which house planets near a boundary appear in.

Which is more accurate, Placidus or Whole Sign?

Both have a long history and their own advocates; there is no consensus among astrologers on this. Placidus is the common language of modern Western astrology, while Whole Sign is the foundation of ancient techniques and Indian astrology. You can look at your chart with both and decide for yourself which one describes your life better.

If I don't know my birth time, is there any point in choosing a house system?

Without a time, the houses and the Ascendant can't be calculated, so the choice doesn't change the result. If you mark on the form that you don't know your birth time, the planets' signs and aspects are still calculated. If you know the dates of important events in your life, you can try finding your likely birth time with birth time rectification.

Are there other house systems, such as Koch and Regiomontanus?

Yes. Many systems have been defined, including Koch, Regiomontanus, Campanus, Alcabitius and Topocentric. 12astro offers the four methods that are most widely used and most clearly distinct from one another: Placidus, Equal House, Whole Sign and Porphyry.

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What are house systems? Placidus, Equal House, Whole Sign and Porphyry